Hermann was not called Fersen because there was anorther general Fersen in the Russian army: Hans Heinrich von Fersen (1739—1800, Ivan Evstafievich Ferzen in Russian) from Livland.

https://ru.wikipedia.org/wiki/%D0%A4%D0%B5%D1%80%D0%B7%D0%B5%D0%BD,_%D0%98%D0%B2%D0%B0%D0%BD_%D0%95%D0%B2%D1%81%D1%82%D0%B0%D1%84%D1%8C%D0%B5%D0%B2%D0%B8%D1%87