This is curious: Hover over the first icon in the Pathfinder panel and the tool tip says "Unite …" Then after you've "united" some shapes, go to the Edit menu and it says "Undo Add." Same thing with the second one, "Minus Front." It shows up as "Subtract" in the Edit menu. Um, why?
That's what they used to be called in an earlier version. (Add to Shape Area and Subtract from Shape Area) Obviously no one has taken the time to update that area of the code recently.
^ That's what I suspected.