My wife and I also noticed that when making a reservation for a stay during the week it looks like there are only a few reservation only sites available. Then when we arrive, there is almost no one else camping in the reserved only sites - and some times we were the only campers.
We have camped mainly at COE and state parks in IL, MO and AR. Of those sites that are available, they're usually closer to the park host. We kind of figured it's more efficient for the park host if the campers are close to their site...
2018 Flagstaff E-Pro 19FBS
2014 Chevy 1500