Prime95/LLR on dualcore CPU - strange thing?

[Cruelty]

Normally I run two copies of Prime95 on my dual-core A64 CPU, so I haven't noticed it before. However due to the recent high temperatures accross Europe I had to switch off one of the copies. As affinity was not set I've noticed under task manager that remaining copy of Prime95 was running on both cores with cumulative CPU load of 50%. Further tests showed that there is no performance difference between running Prime95 with and without affinity being set.
So given the fact that Prime95 calculations at iteration I depend on results from previous calculations at iteration I-1 (LL-test) and the reported complexity of creating multi-threaded version of Prime95, can someone explain it to me?
I figure that this "load-sharing" is an OS feature, but what keeps it from running at 100% on both cores with single copy of Prime95? (which is probably a dumb question anyways )
BTW: the same applies to LLR and PRP.
OS=Windows XP Pro

[Jamiaz]

Basically if there is a non-multi threaded program running on a multi processor computer, the os decideds where to run the threads. In simple terms it runs the thread on a processor, then the next command starts and it goes to the unused processor, and so on and so on, It is basically using 100% of 1 processor, but since there is 2 of them it is jumping between both of them. I have seen this for years using dual prcoessor computers, it is a simple way to tell if a program is multi-threaded or not.


I'm sure someone here has a much more technical description of this, but I like my simple answer. :)

Matt