fplusforairtran - here in Phoenix one of the local news channels did an investigative report on the baggage scales at the airport. Guess what? Most were not "zeroed" properly and many were just wrong. They had a representative from the weights and measures bureau along who did the testing so it was all legit.
If you ever encounter a problem when having a bag weighed ask them to reset the scale or use another. Make sure that the scale reads zero before placing your bag on it. Finally if you feel the scale is still wrong contact he weights and measues bureau. They inspect devices from gas pumps to deli scales to ensure accuracy. Of course it will not solve your immediate problem of having to pay excess baggage fees but if you make a note of the exact scale used and it is later found to be faulty you can request a refund.