Because the ported one was opened up far too much. The tip of the triangle was opened into a square. Take a look;
http://www.cardomain.com/ride/3385930/3
I have a newer M90 but it had been sitting collecting dirt & dust. It had to be cleaned & the grease needs to be replenished. I've got some Oil I can get more from GM if I need it. Just need the grease more.