Jumping back to the screws for a minute, here's a couple of websites that will hopefully explain the the technology of "Thread forming" (to give them their proper name) screws. Self tappers are strictly a whole different ball game.....
gives a pretty picture
is more verbal, but gives you the picture.
Long & short of it is there's thread forms, and there's thread forms. What sticks like lue to plastic or metal may pull straight out of wood & vice versa.
I'l lleave you guys to carry on talking about the wood now.....