An "in universe" explanation"? Um...
Well, in the late TNG/early DS9 era, it was clear that space station personel wore a different uniform to those on a starship. Starship crews were evidently more 'Starfleet formal', whereas base personel were 'Starfleet casual'. But in Voyager
, the ship crew also
wears the DS9 'Starfleet casual' look, having evidently been launched after Starfleet had issued the order saying everybody has to switch over to the other uniforms. So, "Generations" probably depicts the changeover from one to the other (though the old uniform doesn't appear to have been completely phased out at the end).
I do think it was a bad idea changing the uniforms (the movies and Voyager too -- I'd have prefered if they'd stayed consistent and kept different uniform types for base and starship personel). IIRC, the whole point of the DS9 uniform was that it was supposed to be more like something you'd wear in that kind of dressed-down enviroment. It's the kind of uniform an engineer would have on when he's got his sleeves up and is tinkering with components. Maybe Geordi's team should have been wearing them on the Enterprise more often?