Why? What else would cause two ostensibly round-to-start top headset cups to allow fore aft bearing movement but not side to side? I'm really curious.

I pressed in the second cup - went in square and even, no tipping on entry.

I can tell you that the bearing doesn't seem particularly pinched side to side in the cup, just a maybe .75mm slop front to rear. Also, with my digital calipers it was hard to find much difference between front to back and side to side measure of inside of cup or tube itself. Margin of error for actually spanning the widest part and holding the caliper at equal depth on both sides made a difference. But overall it did seem front back measured very slightly larger than side side.

And in the end the bearing that fit perfectly in the brand new cup slides exactly fore and aft. Seems suspicious that a reaming error would just happen to mimic ovalization caused by riding.

So anyway what else might it be?