Boobs are like snowflakes. No two are exactly alike.

Mismatched boobs happen because no one is perfectly symmetrical.
Breast asymmetry can vary from a super-subtle difference to a whole cup size (or more) from boob to boob.

Unfortunately, you can't just exercise the asymmetry away.
Nipple variation is a thing, too.
If there's a drastic difference and it's really bothering you, there are ways to fix it surgically.
