Ever tried to get a reservation at Yosemite, Bahia Hondo State Park, and some other really popular locations?
Heck, I even know one couple who reserve a specific site EVERY weekend all summer on Lake Texoma north of Dallas - and only go about 1/2 the time. So the site is locked out for drive up campers even those weekends because the couple doesn't care about losing the Friday night camping fee.
I knew there were some people who played the system and didn't care about the cost, but after we got into volunteer camp hosting at NPS and state park - I've found it is very common.
I've had 'fully booked' campgrounds less than 1/2 full on prime weekends.
On a regular weekend my unscientific guess is 20-30% of paid campers don't show up. When you see the same names reserving the same sites over and over, and seldom show up, makes me cynical.