I would have to say some sort of spine issue. That is assuming you are not getting fletch contact on your rest. A 10 grn weight difference will not make that much of a difference in impact. I don't think it would make any at that distance to be honest. I don't think spine would make that big of a differance either though. But if they weigh less, I bet the spine is off as well. They may not be as straight as the others either.

Paul