Hi 
I saw that since last update Admob Rewarded Videos taking commission @Diego
I found this in another older topic:
Normally most of the admob rewarded videos are 30 seconds long and everything works fine in my app.
But approximately 1 of 10 Rewarded Videos shown by Admob Rewarded Video Component is only 5 to 15 seconds long and the user get no reward at end of these short videos. Why this happens? Are these the videos, which are shown from Unity for commission?