Somethings been bugging me about the avocados that keep growing my garden, why are they so different in looks than the one I buy in the store?

I had a tree in the backyard and the avocados are perfect to eat, they’re smaller than average avocados sold in stores though but also smaller than the tree. The skin isn’t green like the one from the store, it’s actually black and there’s red specks on it, also it has a bigger pit inside. I’m not sure if it’s the same avocado variety sold in stores but if it is, then why are they so different? I mean I just EAT them, they’re delicious but I’m just curious about why they’re so different, besides the taste there’s no difference. The taste is like an avocado but also a bit different, can’t describe it well.