Frankie says in the Mac ending, that this day was for testing the stairs, but in the Bloo ending, she wants them to race to the store...literally.
Whoever won passed out from dehydration, it cuts to the hospital, winner in the bed, loser in a wheelchair.
Mac ending: Bloo tries to say the race never happened, and Mac was delusional from the dehydration.
Bloo ending: Mac tries to make a technicality that since Bloo was wearing Mac's sneakers, Mac won.
The episode ends with them racing through the hospital either way.