Same experience with Delta Air Lines
Interestingly enough when my son recently checked in at TPA for a Delta flight his bag was 2kg overweight. Delta wanted to charge him $150 excess weight charge. He took more than 2kgs out of his case (for me to carry later on a seperate itinerary in C class) and he re-weighed his case, but on a seperate scale. Oddly it showed that he was still over. We then tried three different scales and discovered that all showed different readings with a discrepancy range of 3Kgs! I immediately called for a Supervisor and politely stated that if they wished to be pedantic about 2kg I challenge the accuracy of their scales and showed him why. The bag was accepted without further comment.
To be honest I would expect a pedantic attitude from LCCs but not from a major airline such as DELTA AIR LINES. They should at least calibrate all their scales to read accurately.