Cécile, I created a report similar to yours with custom columns that are based on the value in score fields, and it did have values in the custom columns for each "Group Leads by" value - including a "0" when there were no leads that met the criteria of the row and column intersection.
Have you tried adding a different custom column to see if you observe the same behavior and/or creating a new Leads by medium report to see if that has the same problem. If so, it's probably time to open a case with tech support.