The Old Mixer wrote:
They were probably just a new design that was being phased in. As DS9 was a spanking-new Starfleet acquisition, they got started with the new uniforms. It doesn't have to be any more complicated than that.
The problem with this idea is when anyone leaves the station, like the time Sisko went to Starfleet Command in "Paradise Lost", they would change into the regular uniform from TNG. Even O'Brien was seen wearing the new DS9 uniform then changed back into the TNG one when he went back to say goodbye to Picard on the Enterprise.