The thinking isn't that you're supposed to offer what you nominate (I usually don't either) but that people need to be offering and/or requesting other people's noms. If you could nom six, offer six, and request six, you'd end up with a situation where a lot of people would nom, offer, and request the same six, and nobody's signups would overlap enough to be matchable.

By making it that you have to offer at least one thing you didn't nominate, and you have to request at least as many as you nominated, in theory nearly everybody is matchable (and if you changed it even more - so that you have to offer twice as many as you nominated, or you have to request more than you nominated - people would overlap even more and be even more matchable.)

But if there's a bunch of extra nominations, that throws the numbers off.