Unexpected refund for (used) E+?

Last weekend I flew from ORD to SIN via HKG. Attempted to use miles + copay to upgrade to BF. I knew it was unlikely based on space, so I paid in advance for an exit row seat in E+ for ORD to HKG (wanting the leg room and a non-middle seat in advance in case there was nothing 'good' left at check-in when, as a silver, I could get E+ for free.) I ended up not getting upgraded for the ORD-HKG segment, but did get upgraded HKG-SIN. I knew I would get the miles+co-pay back for the entire trip, but I did not expect to get the E+ money back, as I used the seat for the longer leg. However, a return was posted to my credit card today from United for the price of the E+ seat, and there's absolutely no other reason I can think of that I would be getting a refund from United.

Any idea why I would've gotten the money back? I'll take it, but it seems like an error..