If I have a number of marks some of which are type unit and some are type exam and I turn on cumulative the average appears to be wrong. It only seems correct if all the mark types are the same. Is that right?
A couple things to check: is the system using the Percentage grade scale as the default? The cumulative averages only currently work on percentage.
Also, have you setup the weightings before creating the markbook columns, or was there existing data when turning on the cumulative average setting? The columns may need to be updated to point to a specific column type after the weightings have been setup.
Hope this helps!