Back in the day, there were very very few JW's that would dare to skip an assembly or convention.
Nowadays, there may be a boatload of 15-minutes-a-month older publishers and barely active ones that don't see the need to give up 3 straight days at a convention.
I think their initial numbers are bloated a bit, but there are also many who won't go to the convention. Further, their count for the conventions is probably the peak on the weekend and that means even more are missing on Friday.