Before a mark is entered for an assessment, its weighting value is not being calculated with the rest of the values in the Markbook. When the mark is entered, the new weighting is applied and all the values adjust accordingly. Marks which contributed more to the average earlier may contribute less once a new assessment is marked and cause their value and the overall average to drop. In these cases, you will notice that if you change the mark from 100% to 0%, the average still adjusts down accordingly. It is only when there is no mark at all and then a mark is entered that you may see the average drop.