Eric, how far out from the house does the deck extend?
I think Tron hit it right on the money. It's possible that the concrete pilings don't go deep enough to hit undisturbed clay. If the poit in which the dug out to dig your basement is further out than where the deck comes out to, then the standard 4 feet may not be enough.
Or they may have just dug down 2 feet and figured "who's going to notice."
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