I would advise to never store data this way if it can be helped. I have seen Access try to subtract 2 values even if they are not numbers. If B can be greater than A, or B can equal B, it stands to reason that there can be an attempt to subtract B from A. If you must, consider setting an unique index and use an autonumber field as the PK.
My guess is that your example is just that and that it isn't exactly what you have but I have to base my answer on what you show.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.