you are not wrong but people dont just shoot an extra time. its damage over time and asuming that the players is keeping them selves alive. which means is if mccree hits 3 shot on you its 3x70 which is gonna get reduced on rein but not on hog, then both gets healed up and mccree hits another 3 shots, so thats again reduced on rein and not on hog.
So over an entire fight just by shooting the hero and not taking shields into account, rein gives ways less ult than hog