Well, here's my uneducated guess. I don't work for Reserve America or anybody else who books RV sites but I have worked in the travel industry where reservation systems were involved.
This guess is based on some language I saw recently on a National Forest Service camping page about walk-ins being allowed to occupy a reservable site for only up to three days maximum at a campground where we sometimes stay.
The on-site park folks often have a batch of walk-in sites they can sell at will and control. Maybe they sell out, maybe not. Then there are sites that, by contract, are held for the reservation system which is funded by the booking fees, maybe including some of the extra desirable ones. Maybe the res system people don't release anything beyond a three day stay to a walkin because they are forecasting getting that booking fee revenue. Maybe they won't, but they want to retain that control in return for providing the service.
So if you walk in and want one of their sites for a week or two, they've lost part of their desirable site "inventory." Again, just guessing here. All of us have the option to book in advance to get the site we want and pay a little premium for it.
Then, too, the online system is open 24 hours and bookings might come in from anywhere at any time. Who knows, maybe there were glitches in the interaction between same day versus advance reservations...a lot of them say you can't book within 24 hours of arrival....or problems with an onsite person not entering the booking into the system instantly, and someone else buying the site online, then arriving to find it full.
This forum has die-hard fans on both sides of the "should I reserve" question. None of us like to pay a $8 fee. Sometimes I do, sometimes not. Just depends on whether it's worth it to be sure I have a desirable site on a weekend in high season versus one by the pump out station and dumpsters...or no site at all.