I am trying to export audit logs for a particular Sharepoint site for the below operations and noticed that the count of records exported through PowerShell command "Search-UnifiedAuditLog" are much less than those downloaded through the browser Audit Log Search page. Any idea why I am seeing so much difference in logs?
https://compliance.microsoft.com/auditlogsearch?viewid=Test%20Tab Operations --> 'PageViewed','FileAccessed','FileDownloaded','SearchQueryPerformed'
Per my test, I could not reproduce your issue, I use the following powershell to check related operations, and the results will not decrease.
In addition, you could use the Office 365 Management Activity API to programmatically download data from the audit log.