Is the slaughter shaft suppose to give different xp?

On completion of the slaughter shaft my friend gained 2 lvls I gained 15% xp. I killed most of 3 rounds because he died at the first. 2K says working as intended. We were both 40 and both with in 3000 xp when he turned in the quest he got 2 lvls. I got 15% xp of a lvl. If it is broke Gearbox needs to ditch 2K. You are your own company leave them in the dust.

Did you complete the Slaughter Shaft for the first time or did you do it for a second (or more) time?

First completion nets the most XP while consecutive completions should reward less.

It was the first time for both of us. We only play these 2 characters together.