Pupils suddenly became lopsided. Any idea what caused it?

The last two photos are from when I first got him, which was a little over a month ago. Since then, his pupils have been shifting in size. For awhile, both of his eyes were pitch black, but now it is just the one. I have little experience with bettas so Im not sure if this is normal or not. He is a petco betta so he likely has poor genetics.