It's best if you can post some examples so we can see them for ourselves and know who the two glads in question are.

Some glads stack a lot of defense, that will reduce your damage. Defensive fighting styles also will reduce damage. Certain weapon types do less damage vs certain armor types. Slashes vs Chain or plate for example deal less damage. Glads who are tanking absorb a lot of damage and reduce hits a lot. Many a rampage will go off at the start of the fight and be a 1 damage hit. I suspect there is something like a "tank health meter" because the more a tank gets punished their ability to absorb damage goes down. If a glads AL is too high that will reduce damage (although you probably make it up with increased attacks). If your BL is low you'll deal less damage. A low attack "roll" or whatever the fight engine does for its RNG can result in a low damage hit.

What glads are you having issues with? Without additional info it's hard to give more specific help.