Neutralizer was stealth-nerfed at some point. The explosions now do less damage than those of Biohazard against neutral targets.
I don't know why anyone working in GH/OOO thought Neutralizer needed a nerf of all things, but it seems weird to me when Bio is the gun that has the status.
Poison and heat level are factors in affecting the damage, check and test again.
Also I think (not sure enough) that some monsters have increased or decreased resistance to some damage types:
not very effective < ??? < normal < ??? < super effective