When you configure a Number of Errors Acessment Form with some weights different from 1, the grade mapping table extends to consider the possible maximum number of errors, as it should. However when you save the form some values configured on the grade mapping table are not saved, only the first values are saved.

Replication steps:

1. Add a Workshop in a course
2. Configure the mandatory information (Workshop name, Introduction and Submission settings) and set Grading strategy to Number of errors and press the Save and Display button
3. Edit the acessment form and define 2 Assertions with something on the description and configure the weights to 2 and 5
   
4. Press Save and Continue editing. Result: The grade mapping table allows grading values for 7 possible errors, which is consistent with the weights 2 and 5 and the resulting possible number of errors: 0, 2, 5 or 7.
5. Select grades for the errors exactly as sugested: 85%, 71%, 57%, 42%, 28%, 14%, 0% and press Save and Continue Editing.
   
6. Go down to the Grade Mapping Table and you will see that only the 4 first values were recorded and the last 3 were ignored.
   

The problems seems to be that only the exact number of Assertions being shown is being recordered, as you can see on the suggested workarround.