Be careful not to compare medieval healers with modern healers. A great leap forward was made in the late 15th, early 16th century (like knowledge about anatomy). The traditional medieval method was, now bluntly generalizing, reproducing knowledge from the Greek & Roman period (the so called auctoritas - meaning "authorities"; this is recurrent in pretty much all of the medieval "science"). The question then is, what do we expect that our healers can do? While today it is evident that a broken leg will not kill you, it might have killed you easily a 1000 years ago due to infections, which (European) mankind was not properly able to treat.
I think it's already nice we have some healers, including those that tend to you when you are wounded. Expecting them to miraculously heal them faster every time again is asking for too much, and the randomness that determines how long it takes for wounds to get healed in game perfectly fit into that picture. Next to that, think about the other side. I am frankly very happy if an enemy noble is wounded for a rather long time, and would be rather upset if he heals quickly.