If you are able to establish a TAC case they most certainly assist you with trouble shooting in the event there is an issue with how things are being reported.
Below is a link to the eLearning of how the Delta is to be viewed.
http://www.cisco.com/E-Learning/bulk/subscribed/SNTC_vods_EN/Inventory_Collection_Delta.mp4
It is correct that you do see the "whole" inventory collected by collection date and then compared. If there have been no changes, that will be reflected.
Please share what you were able to find out from TAC.