Flew FNT-ATL, ATL-SJO (Costa Rica) two days ago. Upgrade cleared on FNT-ATL without certificate attached. Attached regional upgrade certificate to ATL-FNT. I was No. 1 of 7 with 3 empty seats on the morning of departure. All 3 seats supposedly sold between the time I checked in at FNT and landing in ATL. Diamond desk and Delta Twitter said there were no paid or mileage upgrade options. I think either the certificate was incorrectly attached or Delta played games because I just don't believe three people outright bought first-class tickets on day of departure when a paid ticket had prices above $1600. Especially since there were 4 empty comfort-plus seats. There was 1 other diamond and 1 platinum in comfort-plus, judging from bag tags.