Personal Theory: GANTZ gives a different amount of points depending on how strong they are or how important. Tae was very important because she could've let GANTZ's secret out so had a huge bounty on her head. In actuallality Onion Dad was really weak, but because they were new to this and on Kei had the suit it seemed harder than it was. The 8th Grader could've handled it easily from the begining, right? Thus he was only 3 points. The Bird Boss was a flying killing-machine, thusly worth lots of points. On top of that the building that collapsed killed tons of the little chics and Robo-Suit thingies.
And with the force of many bladders god flooded the earth and made it anew.