For me it's a clearance issue. 
Clearing my nose on release.

If my spine is close, due to head weight, the cock in/down ward helps. 

Been shooting cock feather in for 30+ years ever since learning it from Jack Howard. Am RH and use RW and shoot cock feather in no matter the type of bow I'm shooting.

The real true benefits of cock feather in besides clearance is the ability to LOWER ones nocking point substantially. In my case was able to lower nock point by 3/8ths of an inch. One will never know the true benefits of using cock feather in until you lower your nock point as far as possible and still get great flight. Only then you will see the difference when you shoot an arrow the standard way of cock feather out and see that it won't fly with this lower nock point.

I also agree it makes no difference. I don't even look and they all stack it there no matter which way the feathers are pointing

I shoot 3-3" trad vanes cock feather up....Awesome flight. quiet.  love them.   

Quote from: valleysniper on December 12, 2020, 09:37:25 AM
why would shooting cock feather in give better arrow flight ? and, what is the positioning of the fletching ? I do get better arrow flight this way, and my fletching is almost at 3 O'clock, seems as though it would give worse arrow flight....Gary

It has to do with the "archers paradox". Look at some slow mo arrow flite and you will be amazed.
Nock position/indexing can make it more tolerable.
I would just try a different spine arrow, from a buddy. And paper test it to see how stiff it is.
Its almost always stiff.
Sorry if this has been hashed over, just to good of a topic not to post on.