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 erverything than is higher than 4.5 = excelent, more than 3.5 = good.)
I was D: D: D: WTF when I got 3.6. I was sure, I can't get lower than excelent. =(
So, your confidence, charisma and relationships decide about 80% from the result.
And remember, never ever let your teacher think, you don't give a shit about his subject.
I've did a terrible mistake, when I thought leaving lesson in front of everyone, eating fruits and other kinda inapproprirate moves won't affect me, cuz I'm too smart and etc etc.
Nobody can forbide teacher give you grade, which he just want to give.^^