I think it just depends on the child, the scores, the stories you tell, etc. If they get the impression that our children are borderline, they might ask us for more.
Keep in mind that the tests are not designed to accurately measure the tail of the Bell curve. That means that it's possible (though not probable) for a child who's not PG, EG or even HG to have a near-perfect test day and get the scores needed for DYS, even though that child doesn't fit the PG profile that DYS is trying to serve.
I don't think being asked for more materials is the norm, though I don't think it's exactly uncommon either. It happens sometimes.
If you think your application adequately distinguishes your child as PG and not "merely" MG or even HG, then I'd say don't worry about it. If you think your app makes GTness obvious but PG not so much, then I'd say you should add more.
But I'm applying in Feb., too, so what do I know?

That's just how I'm approaching my son's app!