You need to modify your relationships to be:
Asset::__pkAssetID = AssignmentHistory::District Tag
Users::User ID = AssignmentHistory::User ID
Remove the user field from inventory as you don't need it and it appears to be the reason why you see the same user for all records in Assignment History.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained