Hi, Andrea.
This is what makes a mitered square unique. Typically when knitting a flat panel you work from the bottom to the top and the sides stay even. But with a mitered square, you’re actually casting on 2 sides at the same time and working to the opposite corner. Each row of the mitered square bends in the center to create a right angle. You are, in fact, decreasing 2 stitches in the center of each row to create that bend.
I hope this helps!