Well they say it's garbage in, garbage out. The CBO apparently just runs already set-up models, given the data they're given, like trained monkeys. These models are static as well, so the impact of unintended consequences and opportunity cost considerations are ignored. They don't take into account the fact that when you tax something you get less of it.
To me, this is doing a half-assed job of analysis. I would be fired for such shoddy work.