I have had this happen to myself, but I have come to a
conclusion or two about why. Basically, the more you know about something, the less time you have to spend rehashing it. The less you know, the more time you have to spend writing about it and it often shows that you are unfamiliar with the material.
"The first assignment in in my senior year high school Composition class was a two page paper that was supposed to be about holidays." and got
A+...because it is an incredibly familiar subject and has been preached to us since we were born, really. Thus it was easy to throw together in a hurry.
Now, the paper that actually deserved that praise for was a fifteen page introduction to the history, present, and future of cryptography, like my own HO-scale version of Cryptonomicon. While I'm sure you knew a lot about the subject, writing about it was not as familiar to you and there was more to explain. Also, the longer a paper is, the more room there is for little tiny flaws in writing, organization, or logic to appear.
Point number two: Class size in schools matter, too. If an
English teacher has 25 students, there won't be the same amount of attention paid to each paper. If the papers are shorter, then more of it will get read. If everyone is handing in a 15 page paper, then odds are the teacher will just skim it and see what your general
gist is. If the teacher doesn't happen to like or understand the part that he/she read, then you are
screwed.