I just thought of something.
This is an extreme stretch pulled from my tired mind, but it might explain something.
Since Kyurem is from space or whatever, what if he* was, like, a runt or something? Maybe his stats are so low because he's an inferior of their species. Maybe they could do some thing in the third one where, like, a full version comes from space to get it or something.
Like, all the other Kyurems could be super space dragons, and this one fell to earth because it couldn't keep up with the others?
It would kinda explain his incomplete appearance, and low stats. The Kyurem in B/W could be the ugly duckling of Kyurems.
I guess I kinda just want a space themed pokemon game...
* I'm just reffering to it as male for siplicity.