This is mathematically necessary. Suppose the national average is 10 hours a month. Everyone that is below that brings up their time to 10 hours, while those above that keeps it up. The average is now going to climb, maybe to 15 hours. Which becomes the new benchmark. So people bring it up to 15 that were below that, bringing the average to 18. The process repeats. In the meantime, those way above the average see the averages climbing, and will increase to stay ahead of the curve.
When I was a teen I remember our service overseer giving a talk on thursday night and he said "The congregation average hours are 10/month. If you're doing below the average, then you're not doing enough. We need to get everyone in the congregation above the congregation average." I tried to explain this to him after the meeting, but he was just not capable of grasping this math concept. Dumbass.