This is because the Android emulator is a real emulator emulating an ARM instruction set on your x86 box (it uses QEMU), whereas the iPhone Simulator is a proper port of CocoaTouch to Mac OS X.
I think both of them suck. The Android emulator is slow and the iPhone Simulator is inaccurate[1].
Here's an effort to port Android to x86: http://www.android-x86.org. I see a a 100% x86 Android emulator running on VMWare in the near future. Tasty :)
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[1] For example, the iPhone Simulator lets you use NSFileManager to create files and directories inside your app bundle. This does not work on the actual device. I ran into this during a demo :|