By the same way Saitama is just B Class lol.
Hero association ranks people in order of their feats, they could just have not seen FF doing something greater than DS or Bang, or just consider the threats he defeated as not as strong as they should be (such as how when Saitama defeated Lord Boros, yet no one acknowledged his strength because no one knew how strong Boros truly was. How Saitama defeated DSK and since people thought DSK was weakened and that it was "because luck", etc.)
Which in fact makes sense considering his principal atribute is speed and he's a Ninja
I mean. Even if you consider him being above Darkshine as something unrealible, He being stronger than Bang is a fact, since he could match a non-sleeping Garou for a while without his sword (something that weakened him). The same Garou that previously matched Bang at his 100% while sleeping
Well, even Spiral Garou is comparable in physical strength to Darkshine. Then add the evolution Garou had against Bomb, against Bang, and the creation of "his fist" in the 1v1v1 battle, yet FF could match him, endure his attacks and damage him without his main weapon.
Nah, not the same. In DB rankings are by Ki: something that's almost always objective.
In hero association, rankings are based in the feats you've done (and communicated to them), your intelligence (Even after Saitama broke all the records he still was sent to C Class because of his intelligence) and your popularity