My girlfriend does this. She almost always leaves two bites of a meal, or the two last cookies, and then eats it later. She says she's eaten plenty and then puts it away for later. I still don't understand.
My partner just leaves two bites and doesn't eat it later. I also don't understand. She says she's had enough but if the portion were slightly bigger she'd also leave two bites.
For cookies I get it. I don't know if your cookies are tiny but I leave two or even just one.
I'm the same way and I can't tell you why. I've always had a weird aversion to cleaning my plate. Once juice or milk or whatever is below a certain level in the fridge it is also dead to me. No idea why I've been like this my whole life lol
Edit: Thinking back I used to get bullied about my appetite a lot when I was a kid maybe that's why. And yelled at for eating/drinking the last of anything now that I think about it.... Holy shit I think I cracked it wtf lmao