I've had cards come straight out of the pack with edgewear all around the card. It's not uncommon, and that's not really something anyone has control over. Some cards have the top/bottom sandwiched into the pack when it gets sealed, that's much uglier.
My guess is it came out of the pack like that. The only other thing is if the card is capable of rattling inside the case it might have worn down over time.
Considering how many Base Charizards I've seen that are just badly abused because no kid EVER takes proper care of their cards, I think you're lucky. I've seen people with 1st ed Charizards and the cards have every defect possible and you just want to cry.