Just checked mine since I was up here, no use to you but my fronts are 17mm and the rears 24mm. Both sockets fit well.
Hummm Snap-On is one of the highest quality tools in my experience, interesting. How much slop are you seeing? For example a Craftsman 1/2 drive 12mm 6pt impact socket on a 12mm bolt gives me less than 5 degrees of slop. A 13mm socket from the same set gives me over 15 degrees of slop on the same bolt.
I'm not sure what to tell you, I stand by my original statement, no other size makes sense in that range.