Heel slippage generally happens when a shoe is too wide or too deep or both. Rarely do people think about depth when buying shoes. You can have a wide foot without much depth (flat foot), or you can have a wide foot with a lot of depth (high arch). Even if both feet are the same size and width, they wouldn’t be able to wear the same shoe.
Here’s an example of different depth shoes. The shoe on the left is shallow and the one on the right is deep, even though they are the same size and style.
If you experience heel slippage, then you probably need to look at shoes from the side as well as the top. Shallow feet need shallow uppers or they will slip in the heel.