Why is my damage so different?

I'm playing Mass Effect 1 for the second time with an imported character (Infiltrator, level 45).

Sometimes I one-shot every enemy that is in line of sight, but sometimes only their shield lightens up and they take almost no damage - even if I shoot them several times.

Is this a bug or do some enemies have advanced levels?

Summary of my question:

My shots even manage to kill a stationary geth shield and the geth behind the shield (both with full hp), but sometimes the issue as described above happens. I have to use another weapon and shoot several times until the enemy dies.

Solution 1:

enemies do have advanced levels, and some of them get more damage from a specified weapon vs others, and weapon mods impact your damage as well.