CPUs often come with many variants and optional features, and compiling on the target device allows the compiler to identify the feature set of the given CPU and generate optimal code. For example x86 is not "just" x86 - it can have a number of extensions, like MMX, 3DNow!, SSE, SSE2, SSE3, SSE4, AVX, CMPXCHG16B, AES, and so on. Some of those even have sub-types themselves (SSE4.1, SSE4.2, etc), and there are flags to indicate the presence of extra single instructions. That's only the 32-bit type; there's also the 64-bit type. Other architectures like ARM have their own extensions like NEON. So in fact the APKs are likely not all the same, and will vary depending on the specific features available with the CPU model.