The key here is Nature
The second Kyurem you have has a special attack boosting nature, most likely Modest (+sp atk, - atk). Maxed out, a level 100 Kyurem White with a boosting nature has a special attack stat of 482. Such a spread would be typical of a choice set kyurem-w.
As for the other one, it has a special attack neutral nature (most likely timid, but hasty or naive are also possibilities.). A maxed out neutral nature Kyurem W has a special attack stat of 439, just three off 436, This could be because of three things. 1. You didn't max out the evs, being 12 evs, or 3 points, short 2. You don't have perfect IVs or 3. Pokebuilder messed up as it typically does
Well I hope this helps you out.
Source: I played around with Kyurem on the smogon teambuilder