I did LHC with 5 other ppl today and at random moments we were getting 512k per mob instead of 640k (while everyone hitting). Not to mention that i got 1.2 exp coupon today and i didnt see any difference from yday run (not sure if this coupon affect party bonus exp).
What monsters in LHC were you killing? If the EXP was fluctuating between 512k and 640k it sounds like somebody stopped hitting for a short period of time.
The coupon from the Medal quest is affective on the base EXP of the mob, Bearwolves have a low base EXP but high LHC Party Bonus. Therefore the 1.2x coupon at Bearwolves will be negligible, but would be substantial at Zakum. The +25% Fri-Mon bonus EXP applies to base EXP and specifically LHC party bonus EXP, which is why there is a substantial boost to the Bearwolves EXP during those days.