That's correct, and also the medallion system is much to slow for the bartenders to input every transaction at the very time the order was served.
Consequently they accumulate a pile of paper tickets during a busy time and later enter them into the system during a slow period. If there was an error in charging the drink to the wrong pension there's no way to verify it at that point, so errors can and do occur.
It's not that hard to check you account daily and get it corrected as soon as you discover the error.