This is not a glitch; it is working as we intended.
If there was only one set of radio buttons, it would not be clear which selection you wanted to change. When you are selecting the edits for diffs, you always need to select two edits. There is a left side and a right side.
If you wish to change the selection on the left, use the left "dot"; if you want to change the selection on the right, use the right "dot". The "early" edit is the left dot; the "later" edit is the right dot.
There are actually always two dots, but most of the time they hide to prevent you from picking the same dot twice or picking a confusing order.