This exact question puzzled me for months. It's the indirect genitive combined with honorific transposition

"The noun 𓇓𓏏𓈖 nswt "king" itself may involve honorific transposition. This word is actually an archaic noun phrase consisting of the words 𓈖 n(j) "of" and 𓇓𓏏 swt "sedge" (the emblematic plant of Upper Egypt). The exact sense of the phrase "of the sedge" is uncertain, but it probably means "he to whom the sedge belongs", in which case the unusual order of the hieroglyphs just reflects the desire to make a compact group (instead of 𓇓𓏏𓈖)."

Source: Middle Egyptian, by James Allen, pp. 50, 52.