The content search uses the KQL editor with the query -Author:USER1. In eDiscovery/Content search, the minus sign denotes NOT and KQL is case-insensitive for property values. Therefore, this query returns items whose Author is not 'User1'.
File1.docx # Author = User1 # excluded
File2.docx # Author = User2 # included
File3.docx # Author = USER1 (same as User1, case-insensitive) # excluded Result: File2.docx only.
Reference: Microsoft Search (KQL) property restrictions and case-insensitive behavior for queries in content search.