I sometimes see weight differences in the 200-300 lb range and often that is consistent with one specific scale registering heavier than another specific scale.  I'm happy when I unload more than I loaded, I'm concerned with grain loss/hang up issues if I'm light on the receiving end.

I don't know what accuracy public scales are rated at; most break on 20 lbs, which can add up if we were talking pickup load quantities.  Where you have 4 weight events over three seperate scales, I don't know that 300 lbs above or below (on each weight) is that much for the trucker to be concerned about on a 50,000 lb net.

As the trucker, it does not affect my check that much, but if I see a consistent problem I will call it to the attention of the broker, the shipper and the receiver.  The broker appreciates the heads-up and usually directs me to bill them for the highest net.