The hash for a template includes a *lot* more than just the source:

https://github.com/bobthecow/mustache.php/blob/master/src/Mustache/Engine.php#L491-L499

This prevents hash collisions for anything that would change the compiled output, e.g. if the escaper changes, or a new Mustache library version is used, or it's compiled with strict callables or whatever.