Hello,
If I'm understanding your question correctly,
A) You defeated an opponent and it only counted as killing three ninjas, meaning you didn't get the credit for killing four ninjas?
B) You're unhappy with the fact that you fought a level 77 player that was stronger than you and ended up losing, resulting in low rewards for the event?

In the case of scenario (A), you may have fought someone with a partial team of only three ninjas. If this is the case, then, yes, you would only get credit for killing three ninjas, because they would only have three ninjas to kill.

In the case of scenario (B), sometimes you just get unlucky and fight someone of a low level that is powerful. Some people are good at managing their power and level.

Thanks!