I have no idea how their contracts are drawn up. I do know, though, that when an author and a book publisher make a contract, they typically draw it up so that if the author earns out their advance due to sufficient sales, any sales above that amount they get royalties on.
So for example, if the publisher offers a contract for 50,000 copies sold of a book that costs $10, and the author gets 10 percent of the cover price, then they will give them the advance of $50,000 (usually in segments). And if the book sells 70,000 copies they will have met their advance AND will get 10 percent (or sometimes more, depending on the contract) of the extra 20,000 copies sold. So their payment is still hinged on copies sold, even if there is a minimum they make -- because they still keep their $50,000 advance even if only 30,000 copies are sold.
And if they do not earn out their advance in the first place, but they get another book contract in the future, they usually have to still earn out their first advance from sales of the second book if its sales tops its advance. Of course, if you regularly do not earn out your advance, it's less likely the book publisher will give you another contract and advance, or it will be for less.
I would be a little surprised if Kobold and Sasquatch didn't have their advance and future potential earnings tied to sales figures in some way similar to that. But again, I also don't know the specifics of the contract.
It would really not be to your advantage, if you are a singlular author or if you are a small production studio, to accept a contract for one fixed amount regardless of how well the book is sold. You'd usually have a clause that allows you to make more if the book is wildly successful beyond it's expectations and projections.