So look. PyPy compiles bits and pieces of code generating assembler. This assembler then gets to be turned into Javascript (asm.js) which needs to be parsed and compiled. All of this happens at runtime and while you can technically call the second piece of it AOT, it's essentially a double JIT or at least you pay the cost of double jitting. The equivalent in C would be generating C at runtime and sending it through gcc/clang.