A February 2013 "USA Today" article notes that high school grades still do offer indication of a student's potential for college success. However, the same article indicates that many top colleges don't give much weight to GPAs in their admissions selection processes. This gives merit to several arguments as to why students who perform well or poorly in high school may experience different results in college.
Coursework is significantly different in high school and college. This is especially true for students who take a traditional high school curriculum as opposed to advanced, college prep courses. The difference is so important that some colleges compute a student's GPA just in college prep courses when evaluating transcripts. A student who gets all A's and B's in regular high school classes may not develop the study habits or competence to produce similar results as a college freshman. Similarly, a student with B's and C's in college prep courses may get more comfortable over time, and improve grades in college.
Student motivation also plays a role in a number of ways. High school is required and typically free. Some students take it for granted and float by. College is typically paid for and optional: Students usually choose to go to college. Thus, students who didn't take grades seriously in high school may step up when their money is behind the education.
Passion for Subject
A closely related factor is that students have relatively little autonomy to select courses in high school. They do get some elective options, but much of the curriculum is required to meet state guidelines or college prep standards. In college, students have significant autonomy to pick a course of study and to select many classes within a program. Thus, a student who didn't care for the required courses in high school may perk up in a subject he is passionate about. Passion often translates to effort and taking the steps necessary to achieve better grades.
The simple reality is that students often go through personal changes between high school and college. The maturity of a high school freshman or sophomore is quite different from that of a college student. On the other hand, some students perform well in high school, but become overwhelmed by freedom, independence and partying at college. Some students also take on more responsibilities in college, such as work and families, that didn't get in the way of studying in high school.