One day I was bored then I tried to solo Level 126 ( I was level 45), I pushed every mobs into the lava.
It worked, even working on bubbleboy.
But I recieved only 17 exp and 120 gigas.
Last time someone helped me, He was level 121
But I earned 550 exp
Is this a bug?

you get less exp the more you push enemies

also when the lvl 121 helped you it may have been during double or triple exp

The more mobs you push the less EXP you recive.

EDIT: Didn't see the second question, he could have beaten it on the weekend when friday and saturday are 2x EXP , or even sunday which is 3x.

Or thats just the normal level EXP. 

Thanks a lot =D