Maybe Sisko was always supposed to be Bell.
IIRC, the only time we ever see the 'real' Bell is during the 2024 scenes. Nobody in the present (24th century) ever looks at his picture until the episode is all over.
So for all we know, it's a predestination paradox - Sisko was always supposed to go back and become Bell. If anyone had looked up his data entry even before the episode took place, they'd still have seen Sisko.