It's been pretty accurate for me over the last 11 years.
I have had issues in the past counting pieces of gear as backpack weight that I would ultimately wear elsewhere. Hats, gloves, jackets, gaiters, and all sorts of gear I weighed in my pack at home but were worn on my person when I weighed my pack at the Portal. I'm not saying that is what I think happened here, but it happened to me so it might be illuminating to your situation.
