The thing is, Tiers are not exact replica's of each other, where each tier is ''weaker'' than the other, e.g. lower attacking stats or defensive stats for 'mons filling the same role. Due to each 'Mon functioning different than others, tiers can have a big difference. This means that there will also be some sort of Typing border, where as in OU Talonflame is used a lot, people will start running Rock-types to deal with it. as a result, Fighting types will be used more often to deal with those Rock types. and Fighting types have a weakness to Flying. So, say if there are a lot of Fighting and Rock type attacks in OU, Kyurem-B would not do well in that environment, since it would be too hostile for it to survive and function well. But if there are a lot of Water, Grass and Electric type attacks, Kyurem-B Would have an easy time switching in (provided rocks aren't on the field but let's ignore that for now). And Water and Electric moves are used quite often in UU Beta, so as a result, Kyurem-B can do well here, but can't do well in OU. Because of that, he will drop to UU, where he becomes almost too powerful for the tier.
I think this goes the same for other borderline 'mons, like last gen.
