Kelly ~ I'm sorry, honey, but I don't think it's weird UNLESS you had an agreement beforehand not to eat each other's food. Did you have the agreement? Or is it something you just assumed?

Can you guys agree that a certain shelf in the fridge and cabinet are for your food only?

Now if it continues to happen after you've had this conversation, then I think it's weird. You've made it clear that it bugs you and why it does.

It could be that she has never been on a tight budget so it just never occured to her that it could be a problem.