More often than not, the bulk of the grade in a typical college course is awarded during the last month of the semester. Think about it. You might have to turn in a research paper (worth 25% of the course grade), there might be a final exam (25% of the grade), and there might even be a presentation in section (worth half of the 20% class participation grade). Add it all up, and the last month could count a staggering 60% of your grade in the course. This "backloading" of the grade is even more extreme in courses where there's a final that counts very heavily. It's not unheard of for finals to count 30 or even 40% of the course grade. Some schools even have rules forbidding professors from counting the final for more than 50% of the course grade.
You might wonder how professors came up with the brilliant idea of having up to 2/3 of the course grade awarded during the last 1/3 of the semester. Sometimes it's because professors don't want the first piece or pieces of work to count too heavily. They know that some students blow off the first test or, not really knowing what to do, mess up the first paper (or two). Professors don't want these students to be too heavily penalized for their first mis-steps and, more important, do not want to deal with hordes of students who are down on the course after only a month. Finals also weigh heavily in courses where the material is cumulative (think about how much more sophisticated your command of Chinese or Biblical Hebrew or Ancient Greek is in the 15th week of a course than it was in the 4th week). Professors in these sorts of courses do not want the more basic skills learned at the beginning of the course to count as much of the harder ones mastered by the end. And sometimes a course is bottom-heavy simply because the professor is running behind schedule and has postponed the due date for some paper or the date on which some exam is administered....