This thread is mostly answered already, though in summary: old open world rare and epic gears now scale up to the same ilvl as dungeon rares and epics.
Old common gears from open world currently don't, while new common gears from open world scale up correctly.
Old common gears not scaling up correctly is likely a scaling bug, and it's not yet clear if/when blizz will fix this. As for scaling bugs, DF dungeon gears are also scaling incorrectly to ilvl 79 instead of 84. Among other possible scaling bugs that may exist out there.