From the United TA:

"76-seat Aircraft" means aircraft configured with more than 70 passenger seats but no more than 76 passenger seats, and certificated in the United States for 90 or fewer passenger seats and a maximum United States certificated gross takeoff weight of 86,000 or fewer pounds.


I'm sure they can get a MRJ 90 model certificated for 90 seats. If they can't, it's 92 passenger limit is over the top.