That would explain it. I didn't realize some active flight plans were excluded. It seems like FlightAware does pick up some VFR flights that are getting flight following. I ran across this paragraph from Mark Riley's (owner) blog re N929X's VFR flight home from Oshkosh
on July 29, 2011, which is on FlightAware."We climb to 9,500 on course for home. We have 440 nautical miles to go and the [wx] system extends just to our north for most of the way with tops to 50,000 feet. We pick our way between the clouds at 200 to 235 knots for an hour and 45 minutes. The miles slip behind us amazingly quickly at these speeds. Soon we are in Western Pennsylvania with home showing on the far side of the map. We have flight following the whole way, so the flight shows up on Flightaware.com. We are finally confident we will make it all the way."
FlightAware is a great service. Thanks!