some discs fly stable because they are so fast. like can you imagine turning over a wraith your first day out, however the wraith mold isnt stable.
its all about cruise speed, the speed a disc needs to reach for ideal flight. wraiths reach their cruise speed they will turn a while before coming back. If you cant throw the disc fast enough it wont turn over even if its high speed understable.
then theres the teebirds, rocs, predators, wizards ect. that no matter how hard they are thrown, if done so with good form up to most any speed should remain flying stable because the mold of the disc has a stable flight.
Im sure therer are more in depth answers somewhere around here, and I bet they were written by a more articulate person than me, especially at this hour in the morning (Im going to bed this was the last unread topic anyway