How about this:
it's in-game values, tuned for gameplay reasons. Villains' and heroes' stats are different.
You can get a glimpse how potent ME weapons are in cutscenes. They kill in 2-3 shots or a single burst, through the shields, both the protagonists and antagonists. You probably wouldn't want a "logical equity" in-game. To recreate the experience, you'd need a difficulty level where you deal damage like it's easy mode and receive it like it's ME2 insanity.
Enemies don't biotic charge or overload/neural shock/lift/singularity/detonate etc. you either, simply because it would even up the playing field too much towards the "sensible" odds (i.e. nec Hercules contra plures). They compensate by having slightly upped weapon damage, but again - if you demand equity, it would mean that they get your toys as well. You wouldn't want a combo of Marauder overloading you and a Banshee biotic charging you directly, would you?
In short: gameplay reasons, likely tested extensively before release.