I didn't notice the firing pin mark until after 10+ years in the army and many many thousands of rounds fired. It wasn't until I started hunting with my AR that I actually unloaded a round out of the chamber instead of firing it. It looked really strange to me...
It's funny the things you notice even after doing the same thing literally thousands of times.
Anyways, I concur it is normal and nothing to worry about. But just to be safe, always keep your weapon pointed in a safe direction when you chamber a round.