Are you just switching the version of PerformanceTest used, between 32bit and 64bit, or are you also switching between a 32bit and 64bit O/S.
And are these results from different machines or the same machine with a different O/S?
This example is same machine/hardware/os (W7 Ultimate x64) with the latest x86/x64 versions of the test program. When I dual booted windows xp x86 I had the same result with it having much higher 3d complex scores than W7 x64. In the x86 version it appears to use 100% of the GPU but its throttled back for some reason in x64 version. This effected both my current Radeon 6870 and my previous GTX 260.