Shifting Trailer Weight?

As @Dockbumper said it's probably due to you setting your brakes on the scale. The other thing is your position on the scale. If you aren't straight and centered, your weights will be slightly off. If you are farther forward or backwards on the scale than the last weigh, things will be slightly different. Usually it's a matter of 50 - 100 pounds, but if you went "Full Swift" pulling on the scale then I've seen as much as 500 pound difference in gross weight.

This is all assuming you had the vehicle completely on the scale in the first place. I have had to make my trainees GOAL to understand they are not, in fact, on the scale platform. Once at Casey's in Cedar Rapids it took my guy so many attempts to get properly on the scale I was asked if I got paid for the miles I drove in the truckstop.