I've been doing some testing and not sure if this is just coincidence or what but...
When I turn OFF "Get flights from Radarbox Network"  (Network off) then I get more Live (MyFlights) contacts !!  Confused  lol
I've noticed this too.  Although I haven't done any scientific testing, it seems to work like this (I'm sure that AirNav support will put me right if I've got it wrong).
If the network is tracking a flight outside you "live" coverage area, as that track approaches where it will display live, it keeps showing as a network (5 min delayed) track.  I've watch stuff go through my overhead at FL390, well within my live coverage area, and it remains a network track.  I don't know if this is deliberate, but I suppose that it does reduce track jumping as it goes from network, to live and back to network again.
If I deselect network data with such a track showing, it immediately appears as a live track, but  5 minutes ahead of where it was.  And you are right, the number of live tracks goes up, so this must be happening with other potentially live tracks.
Not sure if I like this or not?  I suppose that it does keep relative positions correct.  On the other hand, it might just be a bug!