It's a generalisation not a hard rule. Take an OCD, it's called a fuzz and it certainly prefers to play in that arena...however it can be dialled quite light....it's just a bit tricky trying to hold it there. But essentially it was intended to be a fuzz by design. A Rat is a heavily boosted overdrive circuit. Sure, you can coax a light break up out of it, but that wasn't want it was intended to be when it was designed. However, take something like a woolly mammoth and there are no light overdrive sounds in that particular box. It's all fuzz and nothing else. There are lots of mis-labelled fuzzes and distortions that aren't what they say on the tin too.
Never knew the OCD (fulltone you're referring toright?) Was labeled a fuzz. To my recollection its supposed to be based on the 250.