? It's simple - of the four versions, three are in museums, and one of the more famous and evocative versions (they are all painted in different media) has become available for sale.
It's not that unusual for artists to do more than one version. Van Gogh did quite a few of his iconic 'sunflowers'. More to the point I just can't grasp why a piece of art is worth paying over £100 million for. I could never value anything material that highly.
The painting which has just been sold has an image of Munch standing on the bridge looking desolate, apparently a reflection of his own dreadfully unhappy childhood. This figure is not featured in the other versions, and each copy is painted differently, using differnt painting methods, so each is unique.