If you study the specs on the GM heritage pages you will find that the GVWR will change with different size and ply of the tires. Maybe that is what was keeping the rating lesss than the spec plate states.
I don't know... I believe the original registration papers are required to state the reason for any modified weights, and since it specifically mentions that the reason is to avoid exceeding the axle rating I don't think it could be due to the tires...
