It could be that you had more money than you realised, but that is unlikely.
its just like when they give you two sodas at the vending machine.sure, the owners get jipped a couple of bucks but they make so much money it doesnt even matter.
Thing is, if they find this and find i didn't report it. I can get my account locked.
It still should be their responsibility to get the purchase through 100% without you having to remind them.