I am guessing that it must have something to do with how the feather is located on the shaft. I might do an experiment when I have time and just try the helical clamp on the old jig, shoot the arrow and see if it makes a difference. You can tell they are different, but it doesn't seem like much. The left wing indexer seems to locate the feathers 1/4" to the left of where the straight clamp places them. They are both set up for 3 fletch.
I am thinking it might make more difference with vanes instead of feathers, since feathers are more forgiving, but that is just a theory. It seems strange they would go to the trouble of making a completely different indexer if it didn't have some affect on the finished arrow. I doubt it would be worth it financially, but I don't know for sure.
Ron