Generally double fan means 2 fans attached whereas 2F means 2 single fans that divided naturally that way or was divided. For sellers that don't divide until they get ready to ship, they may not know if the fans are attached or not. I have seen them listed as DF/2F because of that very reason.
And for those who pot theirs up, unless they keep record of what was potted up as 2 single fans, they may not remember and list it as DF
This is just MHO
