Actually, as Chen said, GRs don't care what difficulty you're on. A level 12 GR is the same difficulty whether you open it on T1 or T6. Any discrepancy you experience is likely explained by the GR itself - the types of enemies you were up against, any fixtures that help you harm monsters (the Act V Nephalem Ruins map is rife with these), any shrines that pop up, space between packs etc.