Just adding the missing 20 to the respective attacks bringing it up to 150 and 680 like the other two is a really obvious move though if you look at them.
It explains where the seemingly random 660 stats came from and I don’t see it gaining defense anyway.

Reshiram stats: 100 – 120 – 100 – 150 – 120 – 90 — 680
Zekrom stats: 100 – 150 – 120 – 120 – 100 – 90 — 680

Kyurems: 125 – 130 – 90 – 130 – 90 – 95 — 660
Black: 125 – 150 – 90 – 130 – 90 – 95 — 680
White: 125 – 130 – 90 – 150 – 90 – 95 — 680

Then again, being part Dragon is ultimately to Kyurem’s benefit.

I like when legends have alternate forms, but drastic changes when said Pokemon is part of a larger group is a no-no IMO. There is a balance (not meta-game wise) between Reshiram, Zekrom and Kyurem all sharing the Dragon typing and then having a unique elemental type as well. I feel that balance would be disturbed if Kyurem lost either of its types to accommodate the new one each form seems to represent.