Why does the lām differ between «بِسْمِ اللَّهِ» and «نَارُ اللَّهِ»?

Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules

• Both contain tanwīn
• One has only necessary madd
• A preceding kasrah contrasts with a preceding ḍammah
• The Divine Names are different