I worry about this too. They took into consideration our last year taxes. This year, we are making a bit more (not that after taxes you'd really notice). We only took about half the subsidy, because I knew last year was a really bad year.
How do they get the money? Do they take it from the taxes you pay into this year? The whole thing is so flawed and hard to judge. We got a good basic insurance that is affordable, but the deductible is not and now we have already received notice that is is going to go up about 25% next year. Its like they suck you in and you're stuck.
I'm sure I will be looking for a new plan this year, because the 'wise and knowledgeable' marketplace manager lied to me about this one. And big lies too. While I am happy there is progress (no pre-existing conditions and other things), the whole getting and maintaining insurance is awful.
Maybe we should start having no taxes taken out so they have nothing to take from us in the end. Start saving it ourselves. We have never made as much as we are now, but it seems like the whole check is taxes.