Something you might try that will actually show you how funky carbons can be is after you shoot an arrow rotate the nock and shoot it again ,I've had arrows ( mostly with broadheads) come right in just by re indexing nocks.
Bingo! If you try this with the arrows your shooting you'll notice a difference. Someone on here had a thread awhile back about his homemade spine tester. I believe you'll get some good feedback from that thread on exactly why indexing your nocks will change your arrow flight.
Can anyone post a link to that thread?