Because this is truth.
I had perfect example 2 days ago:
In subjects, which I've not studied, where I counted on my charisma on the exam, I got 4.6\5.0, 4.7\5.0 and 4.8\5.0 grades on each.
Subject I studied, but had not really good relationships with teacher - 3.6\5.0 points.
(In my university...