This can be explained using a simple diagram. Filament bulbs are point sources, that is, the light emanates from almost a single point and goes out radically in all the directions. As light travels only along straight line paths, if obstructed by any object, it creates a shadow. This is true of any source of light whether it is a filament bulb or a fluorescent bulb.
In the case of filament bulb, there is no light ray falling on the shadowed area and so the shadow is sharp. Also there is no chance for the light to diffuse into the shadowed region (even if does, it is very small) in the case of a tube light, the light source is long.
So even if the light from one end of the bulb causes a shadow, there is a possibility for the shadowed area to be lit by a light rays coming from the other end or part of the bulb. Hence the shadow is blurred.
