Agreed, it doesn't matter to the clamp where the slot is located. The issue is that there's 2 or 3 mm of metal around the hole in the hangar (the hole that your clamp's bolt goes through), and the clamp must provide at least that much space between the bolt and the seat post, or the hanger won't fit. Preferably, you want more clearance so that the hanger's not rubbing against the seat tube. If the seat tube slot were at the back of the seat tube, that would have given you about 2mm of extra clearance, which makes a difference. I'm glad for you that your clamp and hangar worked and you didn't have to think about this, but I assure you that not everybody is as fortunate as you.