AdMob Mediation and low fill rates

I have been using AdMob mediation for a few days with just AdMob and AppBrain applift.

The issue is that when i was using just AdMob alone (no mediation) i usually got >95% fill rate but now using mediation, their mediation reports show that i am getting around 75% admob fill rate. On the other side, the AdMob id still shows >95% fill rate. Which one do i have to believe? the normal admob report or the mediation report?

Anyone getting the same issue?