I' d be more likely to agree with Howler' s second guess. The smaller diameter arrow is sitting lower on the rest, not square to the string, and that' s why it shot low.

I don' t know about that particular arrow, but you do need to be aware that not all carbon arrows are all-carbon. Some are carbon composites, like the old Graphlex arrows. Mostly fiberglass and just a little carbon fiber. Not that that' s a ' bad' thing, by any stretch. The composite arrows are heavier than carbons. The ones I' ve got weight almost exactly as much as my 2315' s. But they shoot a heckuvalot better than any all-carbon arrow I' ve tried.