Why did the word "Dhū" (ذو) and "Dhī" (ذي) appear with the same function in Surat Ar-Rahman, despite their difference in two verses?

The reciters (qurra') unanimously agreed to recite "Dhu al-Jalal" (ذو الجلال) with a nominative case in verse 27 of Surat Ar-Rahman, because it is an adjective for "wajh" (face), which is in the nominative case. As for verse 78, the majority of reciters recited "Dhi al-Jalal" (ذي الجلال) with a genitive case, because it is an adjective for "Rabbika" (your Lord), which is in the genitive case. However, Ibn Amir recited it with a nominative case, considering it an adjective for "ism" (name).