Its pronunciation is the same as that of the digraph⟨rz⟩, except that ⟨rz⟩ (unlike ⟨ż⟩) also undergoes devoicing when preceded by a voiceless obstruent. The difference in spelling comes from their historical pronunciations: ż originates from a palatalized/ɡ/ or /z/, while ⟨rz⟩ evolved from a palatalized ⟨r⟩.[1]
^Corbett, Greville; Comrie, Bernard (2003). The Slavonic Languages. Routledge. p. 690. ISBN978-1-136-86137-6. The spelling difference reflects the historical difference between a palatalization of /r/ (for rz) and of /g/ or /z/ (for ż).
^Edward Polański. "Reformy ortografii polskiej – wczoraj, dziś, jutro". Biuletyn Polskiego Towarzystwa Językoznawczego, vol. 60, p. 31. 2004. Warsaw: Energeia. ISSN0032-3802. (in Polish)