The word count thing might explain some of this - if the word count for an essay is, say, 1500 words and a student submits 1000, then there are clearly marks available they're not taking.
This usually comes from 2 things - students missing parts of the answer, or not explaining the answer well enough. It's not enough to mention all the things the marker is looking for, but to explain and show your understanding of it also.
Best of luck in the future!