Manufacturers usually figure driver weight of a 14 year old girl, and use a scale that is probably a little more accurate than the quarry down the road.
I've had my current truck on 3 different scales (rock yard, at the dump, and highway inspection), and the difference was in the hundreds of pounds. I took my previous truck ('97 half ton GMC) to a local $5 scale and it weighed 7200 empty, went to the CAT scale at the truck stop the next day and it was 5900 (me, full fuel, in bed tool box, and fiberglass camper shell). The CAT scale said 6700lb hitched to my trailer, which was still less than the other scale said I was empty.
To have one vary a couple hundred pounds is no big deal (to me). There will be a percentage of error, and a light duty truck will be hard to read because you are pulling onto a scale that has the ability to read in excess of 25k pounds per axle or pad.
