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.
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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.
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Nobody can forbide teacher give you grade, which he just want to give.^^