Re: More Suggestion than Paul McKenna
But this is only true for single model of Apple PC, with Dell filling several of the other top spots. So your anecdote of a Power Mac 8100 is likely to be completely unrelated to these issues - the difference between that machine and modern Apple PCs is far greater than the difference between the Mac on the top spot, and the models lower down the list.
I think we also have to ask, why could the machine make a difference? This is application crashes, not OS crashes, and most application crashes are problems in the code, but this would be the same on all PCs, including Apple ones!
So for those kinds of crashes, either the difference is pure luck, or there is some user-selection bias (e.g., the gaping flaw in the study that it measures crashes per calendar time, not use time, and most people with Apple PCs would likely only being running Windows occasionally; or perhaps they run a far smaller selection of software, which tends to not include random unreliable crap).
Another possibility is that things like incorrect memory access are more likely to result in a crash on some machines, but I would argue that's a *good* thing. Whilst annoying for the user, stopping the application can be better than running off with ill-defined behaviour, which is what the Macs may be doing.
Sometimes crashes can happen due to drivers, which could differ, but that's all I can think of, and that wouldn't be most of application crashes in my experience.