Or it was measured differently. They could have stacked ten of them on a scale at once while you are stacking one at a time.

Or it was measured differently and they used the legally allowed error bars.

Or the kitchen scale was off.

Or there is some missing mass from say dust.

Or they were assholes and knew they could get away with it.

Lots going on and it would be hard to debug.

Nah, it’s probably correct. I work in food industry and it’s pretty much never EXACTLY right. It’s always a few grams over or under, and if the bosses get to choose they choose to have it be under.