If this is the first time this has ever happened to you, your boots are mis marked.They could have mis stamped at the factory. It happens more frequently that folks realise. Take the boots back and first try another pair in that same size. If they are mis matched, those boots will end up at a tent sale this fall.
Folks with a 1/2 size difference usually know it. I know. That's why I try on my bigger foot first.