Originally Posted by Halloweverybody

Obviously no proof, but I believe this is the case. Maybe I am a conspiracy theorist here but I think that there is a much more technical algorithm for complimentary upgrades that Delta simply cannot publish without lots of backlash. If delta always went by the book, Platinums would very very rarely get any upgrades, and lose faith in the system. Also if Diamonds always got upgraded they will have no incentive to ever buy first class. Delta has much incentive to go outside the published system once in a while when Platinums upgrade success on certain routes falls below a certain percent to keep them happy. I also believe that Diamonds traveling with a companion DRASTICALLY reduce their upgrade chances.