I too have noticed a resistance to diaper rash, but that's not to say I don't get it from time to time. I've noticed that if I have an upset stomach and get the runs from it, it seems to be more acidic, and therefore, I'm more likely to get a rash, or if I stay in a messy diaper for an excessive amount of time, like if it happens while I'm asleep earlier in the night and I don't go change, it will also happen then. But those seem to be about the only times I get one. I guess my skin is tougher than most ppls. Urine doesn't seem to bother me at all.
I just thought it was something that came with age, the older you got, the more tough your skin gets.
So I guess it all depends on your skin sensitivity as to how often and how severe the rashes will be.
Congratulations on being more resistive to it. I know from experience that it isn't any fun to get it, and I'm glad too that it doesn't happen often.