Last time I checked, a light sabre doesn't leave a bloody wound behind. It cauterizes.
A lightsaber blade only 'partially' cauterizes, (Ref: Splinter of the Minds Eye') and that only due to the speed f the cut. Also, when one amputates a limb, as Harry did here, there is no way to keep it from bleeding out. There are just too many blood vessels in work.
Reference the cantina scene in the original Star Wars movie.