As others have said, it can be helped by removing the lens hood, but it's not a fault of the camera / lens / flash as I've read many times in the past. Oh and I don't mean that you're implying that.
It's simply a matter of physics. Light travels in a straight line and anything between the flash and whatever is within the field of view of the lens, will cast a shadow. The wider angle shots show it because you're getting more into the picture below the camera. Obviously zooming out a little will eliminate that part of the picture. It would be very difficult to produce the shadow at the longest focal length.
Try mounting a longer length lens with a hood and take a picture at it's widest setting. Then you'll really have some shadow!
That's one of the reasons for the taller flashes on hotshoes and brackets on SLR's.