We measure crowd levels by the average posted wait time at certain attractions.
We’re saying that when the number of people in the park might be lower than average, Disney lowers the capacity of the rides so that the average wait time in line is the same as any other day. Disney does this to save money on labor and ride maintenance.
The upshot is that you’re going to wait about the same time at Space Mountain in January as you will in July, even though 95% of public schools are in session in January, and almost none are in July.