Why is LuaJIT so good?

Question

EDIT: unfortunately LuaJIT was taken out of the comparison in the link below.

This comparison of programming languages shows that LuaJIT has an over tenfold improvement over the normal Lua implementation. Why is the change so big? Is there something specific about Lua that makes it benefit a lot from JIT compilation? Python is dynamically typed and compiled to bytecode as well, so why doesn't PyPy (that has JIT now, I believe) show such a large jump in performance?