As long as we're gonna cherry pick stats let's give them context. Jonas V isn't playing for a few games and Biyombo played 42 minutes that night he got 25 rebs. The game was against Indiana which was without their starter Mahinimi. So they went with Jordan Hill who gives up 20 pounds to Biyombo. Despite all his rebs the game went into OT.
Twice in the past ten games he has also had games with 4 rebs.
He has beaten expectations this year, but that's because the expectations were so low. He's also lucky that neither Anthony Bennet nor Lucas N emerged in Toronto. I would never have bet on that, but that's just me.