Considering there are so many different JavaScript engines (IE, several FireFox options, Chrome's V8, WebKit's engine...), how could ONE exploit affect them all

Considering there are so many different JavaScript engines (IE, several FireFox options, Chrome's V8, WebKit's engine...), how could ONE exploit affect them all? Even taking C code and running it through two different compilers (or the same compiler with different options) will often produce different executable code. So one would assume each JavaScript JIT compiler would produce code differently, and yet there's no comment on what browsers are vulnerable? Did I miss the memo that all of the above compilers are sharing the same JIT code?