Yeah. But that explanation doesn't really hold water because R2 is listed with all his affiliations. But let's call that the exception.
So, let's use your premise...
Ventress, in game, is listed as "Separatist". But she was only a Separatist when she was Count Dooku's apprentice. So why no Sith designation? By your explanation, that would have to be the "time period".
But rather than over complicate it. Wouldn't it be easier to say, if Ventress is the #7 all time Bounty Hunter, maybe we could just correct the mistake and add her in that group?