Shepherd's damage physics and the readout are unreliable, don't trust those.
I have taken 900 dmg as Hans and still had like 40hp left. I've taken 1600 as Hive and just got wounded. The shots bounce and crit randomly and I'm sure there's some weird interaction going on with that, but I've taken some huge numbers and survived, so I would not trust those to be accurate in this particular case.