Well there's your problem right there, you have to join in the full league chaos to see the levels melt away.
I've had this happen as well. Sometimes, there's just not enough people to fill that team yet (if yours is the newest addition to the league). Other times, there have been several teams left unfilled for some reason. Getting stuck with the AFK'ers really botches it if you're there for the XP.
I just take my lowbies and go ToT alone on those days and have much better gains.
I haven't lead a league so any opinion as to why this happens would be only a guess.
Some days you win, some days...not so much.