Sometimes items appear to be 'missing' from reports.
It is highly unlikely there is a calculation or presentation error in AA.
Try checking things like how your reports are grouped (e.g. by classifications) - sometimes assets may have been changed by yourself or another user between reporting periods.
I.e. an asset may have had a classification assigned to it prior to a certain date, then updated to another classification (or removed altogether) by another user.
Also, because there is so much data generated in some reporting, using the .csv report option will provide more data / columns as space is limited in .pdf reports.