I could be wrong, but I believe the traditional two under one over was the result of early style (non-plastic) nocks that did not grip the string. The split finger grip let them keep tension on the arrow, keeping it tight to the string. Now that nocks grip the string many archers have gone to three under. I was told it puts less tension and torgue on the arrow during draw, and therefore results in fewer number of arrows dropping of rests. It also supposedly helps promote a smoother more consistent release since the fingers are touching each other.