The trouble is: The campgrounds are probably not full, but the overlap dates often say "not available" or "Full" if you have dates in a row that are not equal to the dates available.
That's the problem with computer sign-ups, it can't tell you that if you come a day later or a day earlier you can have a site. It only processes exactly what you input to it.
I personally believe if we did at least 50% First Come sites, and an enforceable limit on days, we'd have a much better chance of camping anywhere. The reservation system only works for specific functions - and it leaves out the folks that want to stop-over without making reservation as well. It's a no-win situation for many campers. And is abused enough that is should be eliminated.
We were in Everglades, for example, in the W/E area -- about thirty sites, ALL RESERVED ... but, we stayed in non-HU part. THEN, five sites of the thirty were empty at 7PM in the evening -- the ranger would not allow us to stay in one -- the same five were still empty in the morning, and still empty the next night and morning ... and we departed without ever getting a HU site even tho there were HU sites empty EVERY night we camped.