Please reference this thread in general discussion: Reaper's spread is binary
The above thread is several months old, but I cannot find a reference to it with a quick search of the bug report forums so I’m not certain it was ever posted here.
Both Reaper and D.Va appear to have this problem. When firing at a target so that the pellets partially hit the head and partially hit the body, either every pellet is a critical hit, or no pellets are a critical hit. This leads to sporadic damage spikes that appear to be based primarily on random chance, which is not fun for anyone.
I believe this is a bug. Whether or not this is in fact a bug, I believe that each individual pellet should be able to critically hit, and any pellets that miss the critical area should not be able to critically hit. That would reward Reaper and D.Va for good shot placement maximizing pellet coverage, making them feel better to play as thanks to consistently high damage with each shot and better to play against thanks to the removal of the most extreme damage spikes.