It could be a name used in different Slavonic cultures... In Polish the older word szuma was once used alternatively with szum to mean the monotonous, rustling sound of leaves in a forest. Somene might have acquired this nickname for topopnymic reasons by virtue of hailing from some such locality as Szumin, Szumieś, Szumiąca, Szumanie or Szumiłowo, roughly translatable as Rustleton, Rustleville, Rustlewood, etc.
Assuming some letter such as the 'n' did not get dropped over centuries of recopying, there is a huge difference. Shuman looks to be an Anglicisation of the German occupational surname Schumann (meaning shoemaker or shoe-seller; German also has Schuster and Schuhmacher).
