Drywall or deck screws' unthreaded shanks are not full clearance diameter of the threads. Wood screws' shanks are. Generally, the unthreaded shank length should be at least the thickness of the top/outer supported piece being fastened to the support beneath/behind it. Threads gripping into the supported piece are counterproductive to forcing it against the support to which it is being fastened. If your screws do not have an unthreaded shank long enough, then you should drill a clearance hole in the surface piece to clear the threads. Failing to do this can cause the surface piece to crack. Some drill bits are stepped to automatically drill the pilot, clearance, and counter-sink/bore in one step. Note that drywall doesn't really have this problem, since it won't really hold threads anyway. Their bugle heads are also designed not to need a countersink in soft wood or drywall, whereas conical heads on wood screws in hardwoods do. Note that a drywall or deck screw will cut threads in the drywall or deck board as they are driven in, but once the screw is seated, the threads of the screw will be past it, and sunk entirely into the support.