Maybe it's not for technical reasons. It would be easy technically to have them be interchangeable. But someone who just purchased a stamp, and got one with a 0

Maybe it's not for technical reasons. It would be easy technically to have them be interchangeable. But someone who just purchased a stamp, and got one with a 0 or an 0, might worry about it not being delivered because their 0 might be mistaken for an 0. So maybe it's just nice UX? Just a guess.