Gary,

If the scale says 5050 grams at the Nats for an F3A airplane it would be called legal and the model allowed to compete. If that same scale says 5001 on an AMA model at the Nats the model would be deemed illegal and not allowed to compete. That is the correct interpretation of the current rules in both classes. I'm sorry, but the interpretation as you stated in your post is incorrect.

Here is the wording from the F3A code,

A tolerance of 1% will be allowed for possible inconsistencies in measurement instruments for size,
weight, and voltage unless otherwise stated.

No where does it state that there is no tolerance if the scale is calibrated.

Here are the AMA rules,

4.3: Weight and Size. No model may weigh more than five (5) kilograms (11 pounds) gross, but excluding fuel, ready for takeoff. Electric models are weighed with batteries.

No where there does it state that there is any allowance. Of course, 5kg is 11.02 pounds, not 11.