i do not understand gyp scoping

build/common.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 154 matching lines @@
         'use_goma%': 0,
         'gomadir%': '',
 
         # The system root for cross-compiles. Default: none.
         'sysroot%': '',
         'chroot_cmd%': '',
 
         # The system libdir used for this ABI.
         'system_libdir%': 'lib',
 
+        # Set this to true when building with Clang.
+        # See http://code.google.com/p/chromium/wiki/Clang for details.
+        'clang%': 0,
+
         'conditions': [
           # Ash needs Aura.
           ['use_aura==0', {
             'use_ash%': 0,
           }],
 
           # Set default value of toolkit_views based on OS.
           ['OS=="win" or chromeos==1 or use_aura==1', {
             'toolkit_views%': 1,
           }, {
@@ skipping 42 matching lines @@
           # "arm64" and "both" (meaning a fat binary).
           #
           # TODO(sdefresne): change the default from "arm32" to "both" for
           # "target_subarch" once http://crbug.com/339477 is fixed.
           #
           # TODO(sdefresne): set the "target_arch" to "arm" once compilation
           # of skia has been fixed for simulator. http://crbug.com/342377
           ['OS=="ios"', {
             'target_subarch%': 'arm32',
           }],
+
+          ['OS=="linux"', {
+            # TODO(thakis): This is here to measure perf for a while.
+            'clang%': 1,
+          }],  # OS=="mac"
+          ['OS=="mac" or OS=="ios"', {
+            'clang%': 1,
+          }]
         ],
       },
 
       # Copy conditionally-set variables out one scope.
       'chromeos%': '<(chromeos)',
       'chromecast%': '<(chromecast)',
       'host_arch%': '<(host_arch)',
       'target_arch%': '<(target_arch)',
       'target_subarch%': '<(target_subarch)',
       'toolkit_views%': '<(toolkit_views)',
@@ skipping 13 matching lines @@
       'use_goma%': '<(use_goma)',
       'gomadir%': '<(gomadir)',
       'enable_app_list%': '<(enable_app_list)',
       'use_default_render_theme%': '<(use_default_render_theme)',
       'buildtype%': '<(buildtype)',
       'branding%': '<(branding)',
       'arm_version%': '<(arm_version)',
       'sysroot%': '<(sysroot)',
       'chroot_cmd%': '<(chroot_cmd)',
       'system_libdir%': '<(system_libdir)',
+      'clang%': '<(clang)',
 
       # Set to 1 to enable fast builds. Set to 2 for even faster builds
       # (it disables debug info for fastest compilation - only for use
       # on compile-only bots).
       'fastbuild%': 0,
 
       # Set to 1 to not store any build metadata (this isn't working yet but
       # this flag will help us to get there). See http://crbug.com/314403.
       # TODO(sebmarchand): Update this comment once this flag guarantee that
       #     there's no build metadata in the build artifacts.
@@ skipping 243 matching lines @@
       #
       # This variable may be explicitly set to 1 (enabled) or 0
       # (disabled) in ~/.gyp/include.gypi or via the GYP command line.
       # This setting will override the default.
       #
       # See
       # http://code.google.com/p/chromium/wiki/WindowsPrecompiledHeaders
       # for details.
       'chromium_win_pch%': 0,
 
-      # Set this to true when building with Clang.
-      # See http://code.google.com/p/chromium/wiki/Clang for details.
-      'clang%': 0,
-
       # Enable plug-in installation by default.
       'enable_plugin_installation%': 1,
 
       # Specifies whether to use canvas_skia.cc in place of platform
       # specific implementations of gfx::Canvas. Affects text drawing in the
       # Chrome UI.
       # TODO(asvitkine): Enable this on all platforms and delete this flag.
       #                  http://crbug.com/105550
       'use_canvas_skia%': 0,
 
@@ skipping 1223 matching lines @@
             'enable_mpeg2ts_stream_parser%': 1,
             'video_hole%': 1,
           }],
         ],
       }],
       ['android_webview_build==1', {
         # When building the WebView in the Android tree, jarjar will remap all
         # the class names, so the JNI generator needs to know this.
         'jni_generator_jarjar_file': '../android_webview/build/jarjar-rules.txt',
       }],
-      ['OS=="linux"', {
-        # TODO(thakis): This is here to measure perf for a while.
-        'clang%': 1,
-      }],  # OS=="mac"
       ['OS=="mac"', {
         'conditions': [
           # All Chrome builds have breakpad symbols, but only process the
           # symbols from official builds.
           ['(branding=="Chrome" and buildtype=="Official")', {
             'mac_strip_release%': 1,
           }],
         ],
       }],  # OS=="mac"
       ['OS=="mac" or OS=="ios"', {
-        'clang%': 1,
-
         'variables': {
           # Mac OS X SDK and deployment target support.  The SDK identifies
           # the version of the system headers that will be used, and
           # corresponds to the MAC_OS_X_VERSION_MAX_ALLOWED compile-time
           # macro.  "Maximum allowed" refers to the operating system version
           # whose APIs are available in the headers.  The deployment target
           # identifies the minimum system version that the built products are
           # expected to function on.  It corresponds to the
           # MAC_OS_X_VERSION_MIN_REQUIRED compile-time macro.  To ensure these
           # macros are available, #include <AvailabilityMacros.h>.  Additional
@@ skipping 3798 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }