It doesn't have to be old growth lumber. It has to do with the direction of the grain. If you look at the grain rings you can see that the wood grain is not the same direction. So across the grain from side to side shrinks differently than from the center out. So when you look at the tree rings and they are not the same on each side of the board you know that one will cup twist or bow. All depends which way the grain runs.
And that does not even take into account the grain be cut from end to end differently. That leads to even more issues.
But they do care about grain direction because you lose wood cutting for grain direction.
Anyways with out pictures it is hard to describe. But if you use wood with good grain structure in the right places you can save a lot of problems latter. But it is almost impossible to get wood like that out of the stack.
i know what you're talking about...
that's kinda what i was getting at,the trees they cut are small so they have to maximize and that means fewer prime cuts...