I'm not 100% sure on this, but basically I've had a similar experience, and generally what causes the discrepancy is 1 of 2 things.
Either, the mobs were recently aggro'd by or claimed by someone with a higher Master Level than you, and the game though it resets their HP and level, still has them tagged as hit by an ML50.
OR
A character in your party is set to a lower Limbus level than you, and has aggro'd or gotten first action on them first, causing them to be a lower level than you have set on your main.
I see the former situation whenever a multi-boxer or an actual party chains the whole zone to the porter. They kill what they need and go up. The mobs deaggro, and reaggro onto me at 135, but I get lower Exemplar because they were hit/touched by ML50s.
I see the latter situation when I'm in a party with someone who forgot to switch their level back up, and they aggro something or pull instead of me.