Getting less and less Exp. Points for Spartan Ops?

Do Spartan Ops missions begin to pay less Exp. Points the more you beat them on a particular Difficulty Setting? I ask because today I was grinding through the same mission several times in a row on Normal, just trying to kill Covies and fill the Commendation requirements, and it was paying the normal base rate of 2966 Exp. Points like clockwork. During the last couple times through I was trying to get through Ep. 5 Ch. 4, the one where you defend the generators with a Mantis, as quick as possible, but still killing every enemy, even taking the extra time to blow up the Phantoms. However, I notice the the Exp. payout dropped to 2800+ on a particular finish, even though I’d done all the same kills that I had been doing. So I tried it again, finish in pretty much the same time with the same kills, and the payout dropped again to 2700+. Just wondering if this is normal? If it is some kind of daily limit thing? Is it temporary? Or a permanent limiter to try and force you to higher difficulty, prevent farming, encourage you to move to War Games?