When changing the maximum grades on a gradebook item you need to specify whether or not this will cause existing percentage grades to change as well.

If this is set to 'Yes', any existing grades will be rescaled so that the percentage grade remains the same.

For example, if this option is set to 'Yes', changing the maximum grade on an item from 10 to 20 would cause a grade of 6/10 (60%) to be rescaled to 12/20 (60%). With this option set to 'No', the grade would change from 6/10 (60%) to 6/20 (30%), requiring manual adjustment of the grade items to ensure correct scores.