1st time using Cat Scales, need help analyzing results

".......why isn't vehicle GVWR the sum of adding the 2 axle ratings?"

You are not the only one who asks that question, lots of people want them to be equal, but they are not. GVWR takes into account a lot of factors including frames, brakes, etc. It would be fairly easy to throw a HD rear axle under a light duty pickup if that is all it took to get a big rating.