The current matchmaking system allows a maximum of 15 level difference. If you are the unlucky person that is on the low end of this you not only receive a significant amount of extra damage while doing less yourself but you are also penalized with ~ -25% EXP per boss.
Example:
As a Lv178 Character with a teamlevel of 14 a Lv180 boss is worth 245k EXP.
A Lv193 Boss on the other hand will only reward me with 190k EXP
