I've had similar experiences before with bags over 50 lbs. One of the screens on the e-ticket machine asks if any of your bags are overweight, and this is where you're supposed to click "yes" in order to avoid this situation. I think it's misleading; if my weight allowance is 70 lbs. instead of 50, then technically a 55 lb. bag shouldn't be considered "overweight". I tried explaining this to an agent once (I thought she was about to charge me overweight fees) and I didn't get very far. C'est la vie.