Well … I never drove any tipper which had load guages, but i did get loaded by the front loading scoops at quarries and the like. If I asked for 20ton, then the operator would typically load up a scoop, hold it still and record the weight before putting it in. Then on the last scoop (typically the 3rd scoop) , they would adjust what was in the scoop by very very little amounts to get the total correct. The amount shoogled off the bucket could be as little as a few spade fulls. So for a single bucket load of about 7tons, shoogling off a few kgs must mean the scales are quite precise in their measurements.
That was 9 years ago, so based on that, in theory, no reason to suspect that the lorry couldn’t be as accurate as is being reported.