I think different households run differently depending on the personalities of the people involved.
My DP is a big guy - think rugby player type build - and he has a HUGE appetite. He's also rather partial to piling into treats/junk food rather than making himself lunch.
If we had the approach of "all food is for everyone", there would never be anything nice left for anyone else. So staples, like bread, eggs, milk, cereal etc, can be used by anyone, but anything vaguely nice or special - including yoghurts, crisps etc - "belong" to certain people. No one eats the food belonging to other people without asking.
I have teens, both autistic, and I'm also autistic and ADHD. I think for all of us, knowing exactly what we can eat without accidentally upsetting someone else really works.
It also helps with meal planning, primarily lunches. Dinner stuff is just shared between us all. If I know I've got enough yoghurt left for my lunches, I'd be annoyed to find out someone else had gobbled a chunk of it up so I didn't have enough for what I wanted.
Having said that, a dessertspoon full of yoghurt isn't very much but it sounds more about the principle. You'd eaten your yogurt and your DH saw you about to tuck into your DD's yoghurt and he knew that he hadn't ordered more because what was left was sufficient. I think maybe it's just the idea that it's not fair or right to eat someone else's food just because they've eaten theirs slower than you.
Honestly not trying to be unkind, just trying to explain a different way of looking at things.
On reflection, if my DD or DS ate my yoghurt I wouldn't care very much, but if my DP did, I would. That's because he's a nightmare for eating things that belong to other people and it makes me cross that I have to stop him doing it - do you have a habit of doing it as well OP? You don't need to answer here, I'm just asking the question. If you're ADHD you might be impulsive, and you say you normally like your food, do you have a history of eating things that were earmarked for others after you've already eaten your own? Just wondering if that could be playing into it.