Credit hours are bullsh|t. They assign whatever value they feel like.
I took a General Biology class at a local college when I was in high school. It was worth 4 credit hours. I spent 3 and a half hours in class each week. Outside of class, I dedicated 0 hours to additional work. I got A's on all the exams. Our exams were in class.
Where I go to school now, I took the same course. I only took one semester of Bio at the local college, and I needed to have 2 since I was premed. Since the sequence of materials covered was different at the two schools, I couldn't use the credit from the first school. We use the same textbook, though, so it's a good comparision. This lecture course was worth 2 credits, but we spent 2 hours 30 minutes in class each week. We covered material about 5 times as fast. Outside of class, I probably spent 4 hours each week studying for this class. I barely managed an A- both semesters. Oh yeah, and the exams were outside of regular class time.
For General Chemistry, it was 4 credit hours. 2 hours in class. 3.5-4 in lab. Maybe 2-3 doing lab reports, 2 doing problem sets, and another 1-2 on average studying for exams. Again, exams outside of regular class time.
Then again, if you take an English course at my school that's worth 4 hours, you might spend only 2 in class, and another 3 doing readings/papers. No exams.
The worth of credit hours varies a lot from school to school, and department to department.