Hyper EXP Exhaustion period calculating incorrectly

Ah hopefully im the only one who has had this happen, or maybe i have misunderstood, although i dont think so.

I used Hyper Stones for the third time yesterday, this set of 5 for 12 x exp for one hour was started 2 hours before the previous exhaustion time was ended. ( I really dont think this is a factor but im adding what info i can )

Hyper Exp went smoothly, gained a good amount of exp.
Buffer Time also appeared to go smoothly although i didnt really pay too much attention as i was busy in faction.

The problem came when we went to Brimstone, myself and my partner who is also level 89.
Now i understand that in exhaustion i will lose 10% of the normal exp, and having another level 89 char in squad i would say is pretty good pointer of what the normal exp would be

The problem came when he told me how much exp he gained for the previous kill, i checked mine and it was just over half of what he had gained.

Now i cant remember all the different numbers correctly, but one example i do remember was that i only gained 640 exp when he gained 1164. And the amounts were all the same kind of difference as we continued.

We ran a couple of checks ourselves, we tried a mob outside of an instance, that was the same scenario. We left squad, and both killed the same normal no extra stat mobs, same scenario.

Im very wary now of buying any more stones , and i was really wanting to work extra over the next couple of days so that myself and my partner could reach level 90 at the same time without him waiting around too much.

Has anyone else experienced this problem??