Can't be. Normal Kyurem has 660. Absorbing either dragon give it +40. Absorbing the other should do the same, putting it at 740. Although, if it's a "more than the sum of its parts" scenario, it could be higher still. But it can't be lower.
Or instead they add 20. Zekrom/Reshiram are only at 680, Black/White Kyurem are at 700, just add another 20 and call it a day.