Unsurprisingly, tonight's choice of bonus round categories were completely different than last night's. My presumption is that each week has its own complete set of 15 bonus puzzles and that the 10 that don't get used get put back in the pile for potential use at a later date. I also suspect that you still may have a category show up 2 or 3 times in a week, but be different puzzles (not that we'll notice). If the staff can do 15 unique categories for a week, that's pretty hardcore.