I hadn't fed the pet shown above for a few days. When I did today, somehow her hunger reached 10/10 when I had only fed her 8 items, and I'm sure it was 0/10 before I started. This happened to me sometime last week as well, and I thought it was just because I'd still fed her the previous day, but not this time. I've had to use the cleansing pools so I could feed the last two items (I've started feeding all pets except my main one 10 at a time). So is there some sort of error that's causing this?

some items count for 2 points instead of 1. But I don't know if there's a list of which ones do it or if it's random or what.

I did wonder that, and maybe I wasn't fully paying attention, but there isn't really anything that makes it clear which items do that, or if it's random?

I think it has to do with the food's rarity? Especially with "rares" and above.