It's possible that Windows 64 wasn't correctly identifying the Intel chip as 64 bit. Programs that identify hardware are not always very bright, and you can't always identify things made after you write your code.
A couple decades ago, in the DOS era, I tried to identify the status of printers by their response to instructions. I was basically looking for simple things like power off, out of paper, offline. It was a mess. Even though the return codes were well defined, the different brands and models didn't follow the standards.
But the Intel EM64T did work in other 64bit benchmarks... If you look at the Windows Media Player benchmarks EM64T does indeed benefit from 64bit VISTA... just Windows Movie Maker had a problem finding a benefit from it..