I found an egg while adventuring and it's only 5 hours old and it's already at 41%, and I have another egg that is about a day old and is only at 33%, why?

Not all eggs start at the same baseline. When you find an egg/pet in the wild, they may or may not start from 0%.

It's kind of like how all generated pets are adolescents, or if you have referral points, you can choose to make the eggs you breed to start at a later incubation point to cut down a few hours of waiting to hatch. The game just randomly chooses a starting point for wild pets/eggs.