Suppose you had one legitimate transaction, and one fraudulent transaction. Now, suppose the bank had certain evidence to show that the legitimate transaction is legitimate that is sufficient from a legal perspective to refuse your refund request. Then, suppose the bank had no such evidence to show for the fraudulent transaction.
And now pay attention: THE FACT THAT THEY DON'T HAVE EVIDENCE FOR ONE OF THOSE THAT THEY DO HAVE FOR THE OTHER MEANS THAT THE BANK CAN DISTINGUISH THEM. Got it?
I was talking about cases where the bank is NOT ABLE TO DISTINGUISH an actually fraudulent transaction from a legitimate one. Your response "but if they are able to distinguish them, they have to refund you!11" is just completely irrelevant to the point that I was making.