It's slightly silly to be replying to my own query but I forgot to add some pertinent information. Mac files generally consist of two or more forks (no more than two in pre-OSX). I only compute hash values for the data fork. I never include bytes from the resource fork. This is mainly a cross platform issue but there is genuine ambiguity. The resource map at the end of each resource fork has several bytes just before it that are truly indeterminate. Changing them to any value makes no effective change in the file except when the resource fork is accessed while bypassing the resource manager. So there would be an entire array of hash values that would effectively all refer to a single file.

I apologize if this is mainly confusing, but the summary is that I'm excluding all but the data fork from hash calculations on the Mac.