You also have to keep in mind that an older computer will have a lot more background noise in terms of processing, hard drive access (read/write), memory and general cleanliness (dust on the hardware). Three brand new computers using identical parts except for each of those processors might (and probably will) show very different results to those you posted.
I don't think so. Dust is irrelevant unless there's so much of it that it's causing thermal throttling, disk access is irrelevant since Dwarf Fortress is running entirely out of system memory unless you have so little of it that it's swapping, and memory itself, well, the entire point is to see how different systems behave; my current system has DDR400, most Core 2's have DDR667 or 800, all Core i7's have DDR3. You could run a Phenom II, a Core 2 and a i7 using DDR3 if you really want, but most people aren't going to find that terribly interesting since that probably means they've already upgraded recently
Background processes are not particularly relevant, since even the most crufty system isn't going to be spending more than a couple of percent on them, and I'm quite sure none of the systems involved were *that* crufty.