The Discovered Devices report will show how many computers were discovered on the date of a network discovery, then later after another network discovery the Discovered Devices report will show much fewer of those computers for the same date range.
The Discovered Devices report is working as designed; every time a network discovery runs it will rediscover all the active computers that it scans and update the discovery date for those computers. The original discovery dates will be lost and the computers will no longer appear in the Discovered Devices report for their original discovery dates.
An approximate solution to view when computers were originally discovered is to view their created date. Import the attached report (Symantec does not support this custom report) or modify the following SQL query to view this information:
select Name, CreatedDate from vItem where ClassGuid like '539626D8-A35A-47EB-8B4A-64D3DA110D01'
SMP 7.1, SP1, SP2, MP1
Imported Document ID: TECH209020
Custom report to view when computers were originally discovered