Have you asked her why?
My son does some weird things from time to time, but he always has a reason, even if it's not one I really understand. If he doesn't want to say, his new standard answer is it's complicated.
Maybe she thinks it's frugal or that you shouldn't waste things.
Maybe it's a hoarding type thing?
As long as the food isn't rotten or spoiled, it probably won't hurt her even if it's distasteful to other people.
I try not to really react to much and take as much as I can in stride, easier said than done a lot of time, but it doesn't seem to help if I get worked up about things.