Yeah, I’ve heard other people say that as well. I haven’t noticed it myself, but others have said the same. I have no idea what’s causing that.
But, as far as the OP is concerned, nope the CPU load is distributed appropriately. The purpose of the other CPU’s is for other programs. Granted there are some applications that can use multi-threading more optimized, but flight sim isn’t one of them. It’s gotten better than it used to be, but you’ll NEVER see all cores used evenly.