Yeah that would make sense. I don't have any arranged overdraft but my balance can go negative by a small amount in which case it will be displayed. The most likely explanation does seem to be that they are just displaying my balance in a weird way - but still, very odd to show a transaction for a different amount than was requested. And having spoken to them, they see exactly the same thing I do in whatever systems they can access, so it's not just how it's represented to the customer.