Speed calibration is way off! - Killer Instinct Lumix Speedring 1.5-5x32 IR-E Crossbow Scope

I bought this for my girlfriend's Barnett Explorer XP370. I was thinking the "Speed Ring" was separate from the magnification Ring and realized I was wrong when I finally got it. So, trying to make due anyway I mount it and begin to sight in and let's just say the manufacturer instructions suck they are very vague,but I've done this before and figured it should be easy. I shot the crossbow over my chronograph and found it was shooting 358 FPS so I set the speed ring and have at it and sight in at 20 yards. Now they say it is calibrated using a 400 grain bolts and I was shooting 415 grain bolts so I would love to know why when the crossbow is shooting 358 FPS why I have to set the dang speed ring for 290 FPS to be able to hit at 40 yards. It should not be off that dang much, now I realize I'm shooting a slightly heavier bolt but it shouldn't be that far off with the speed calibration. Just saying I appreciate the idea of a "Speed Ring" but take it back to the drawing board and work the kinks out of the system first.