The story I heard from a well known now deceased JD author of magazine/books that had contacts with JD engineers was this.  During testing, the 105 would consistently out cut the 6600.  But one day for some reason, don't know of it was the conditions or maybe the operators, or all the planets lined up right, or something else, but they got the 6600 to match the 105.  So the JD marketing department ran with that.  Honestly, just looking at the specs, there is no reason a 6600 should out cut or even match a 105.  The 105 has just has more separation area vs a 6600 which is kind of a big deal in small grains.  The concave on a 6600 is just a hair longer then a 105 but that doesn't make up for it being 6" narrower.  Same thing with the walkers.  I also have a 6601 brochure and it states you could pull the 6601 one gear faster vs the 106.