He meant the lvl of the extra players he gets in the party.
His main (let's say, lv100) + his noob (lv20) = 100+20= 120 120/2 = 60
His main + noob + lv5 char = 100+20+5 = 125, 125/3 = 41.6...67
The lower the other chars are, more SP you get. Best thing would be to get plain lv1 chars all the time, but it's kinda impossible