They come up with their "IBO rating" by using a bow set at 70 lbs with a 30" draw length, a 350 grain arrow and absolutely no string accessories. Anything that varies from that setup will give a different speed.

Less draw weight = less speed. Shorter draw length = less speed. Heavier arrow = less speed. Peep sight, string silencers, nocking point/loop = less speed.

Your bow is set up with 62 lbs (-8 lbs) and 29" draw (-1"). Your arrows are 8.7 grains per inch so they probably weigh in the neighborhood of 430 grains (+80 grains or so). Then subtract @ 2-5 fps for each accessory you have on your string. Actually, at 250 fps your bow is cooking along pretty darn good.

I assume you've got the 55-70 size arrows? You can try the 125's and see how they do. They could be pushing the envelope for those arrows, but you never know until you try.