Sometimes the transponder can crap out and you have to get a new one. I found out due to getting toll bills in the mail when I had a transponder, but this was pre-covid so they weren't automatically linking the plate number to the transponder for V-Toll on the E-ZPass statement. I also had an out-of-state transponder (Mass Pass) and didn't have to change it until it stopped registering years after moving back to PA.

You can contact them to make sure it's being read accurately and have them tell you what the correct toll should be. Then you'll be able to figure out if it was the transponder or calculator.