So, I think you've identified the culprit. I had them checked for length initially, but just did a rough measure (I don't own a playing length ruler) and the 7i is definitely closer in length to the 6i than to the 8i. I'd guess it's around 1/8" off, which would explain the swingweight discrepancy. Idk if the grip wasn't pushed down all the way, but I'll see when they're regripped. In any case, I'll be gripping down slightly on the 7i now!
Thank you.