I go across several different scales here rather frequently in the Seattle area, rarely am I off more than 200 lbs one way or the other, most likely cause is 30-50 gal swing in diesel in the two trucks I drive across the scales.
So I doubt the scales are that far off. Along with most truckers, myself included, generally know who much there truck/truck and trailer weigh with in a small variance as mine are.
I think Alex hit the majority of the reasons you are if true, 10K lbs over. Then again, look in the bilge, maybe some rain water or equal got in there along with way, a lot of fuel...........major number of ropes hidden in a storage area you did not remove............
She drives me boat,
I drives me dinghy!