It seems that the logic is to prioritize eating such that at least one item of each kind is preserved. So if you have multiple onions (all in garden) and a lone berry elsewhere, the vils will eat onions because (it thinks) you may want to preserve the berry for farming.
I would understand, but I think I had like 5-6 carrots and they kept just eating random stuff, even though I also had a stew ready. Some way to force them to eat some food would be nice. Maybe let them eat the thing they are closest to. Its quite frustrating that when you try to make stew they keep eating unions from your gardens.