I'm not trying to convince anyone of anything, just stating facts. If it doesn't leak with the same washer then it didn't need a new one.
If you're dead set on always replacing the washer, then you don't really have evidence to the contrary, are just repeating something you heard somewhere? Crush washers that don't deform much if at all, because they aren't torqued as much, and aren't holding a lot of pressure, can be reused. Someone mentioned whether they were even crush washers and I suppose not, if not torqued down enough to deform substantially.
Any theories otherwise would ignore their purpose which is to seal so there isn't a leak, which I accomplish reusing them. Oil pans are not brake calipers, which have crush washers that I do always replace. It's true that I'm not a pro oil changer, but I've been doing it over 40 years so I think I've gotten the hang of it.