Some of it comes in some sort of order. GSC/HGSS happens after RBY/FRLG. BW2 happens after BW. XY happens after all of them, presumably, because it mentions events from them. DPPt and RSE are all the same story, told in different ways. ORAS happens after/during XY because it mentions AZ and Lumiose City. However, ORAS is in an alternate universe frome RSE (therefore XY is as well). So, either RSE is in a seperate dimension to the rest of the games, or XY/ORAS is. Imo there's only one answer to this.
Wibbly wobbly timey wimey stuff.