What's the difference between Calculated Weight and Contribution to course total??"
Contribution to Course Total is what percent is earned toward a final grade with a particular grade item.
Please note the difference between this and Calculated Weight: Calculated Weight is the potential, while Contribution is what has actually been earned.
So if an item is worth 10% of the final grade, and a mark of 50% is achieved by a particular student, that student will have added only 5% to his final course grade.