Safari is able to cheat by interfacing more with the OS than V8, which has to be able to run on more platforms. It's the same reason IE beats it out in certain benchmarks.
To be honest, this makes near 0 difference in javascript performance, which is entirely good codegen (and making things like DOM traversal really fast).
In fact, for average website javascript, the fastest "not horrible" codegen will win, because they run for such short times.
