If an ATM dispenses less than it’s supposed to (i.e. They were supposed to get $260 but only got $240) then this is most likely because there was only exactly enough cash to fulfill the order in the dispenser and one of the bills got rejected.
The ATM would send an error alert about it (the error on entropy would be shown as “Dispenser Failure”) and the reason for this could be that the machine is loaded with less money than what entropy has listed. It could also be that there’s a note (a bill) in the rejection bin.
It is also common that people will receive less than they sent due to the difference in prices between their wallet and the exchange we use. (For example, if they sent $200 in bitcoin they may only receive $180 due to our rate being different than what their wallet uses).