i do not understand gyp scoping

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 44ef70aa8d9303ec53b65c55a2818808c5aa1489..44ba09eeb894d5e6895cc8021eeabb3e50296dcd 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -172,6 +172,10 @@
         # 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', {
@@ -234,6 +238,14 @@
           ['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,
+          }]
         ],
       },
 
@@ -267,6 +279,7 @@
       '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
@@ -530,10 +543,6 @@
       # 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,
 
@@ -1777,10 +1786,6 @@
         # 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
@@ -1791,8 +1796,6 @@
         ],
       }],  # 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