This is currently the normal behaviour for the GPO Audit Report. The additional columns in the Audit report, Attribute Changed, Before Value, After Value, are not currently utilized for GPO changes. These columns are only populated with data when there are attribute changes made to an object such as user objects. The Reports use a common layout and design so for some reports, different columns are not valid or used.
With GPO changes, you need to double click each event and click Event Details to get more information (similar to what you see when viewing the event log entry directly on the DC.
Enhancement request VSTS010118 has been submitted to Development for consideration in a future release of Active Administrator.