Why does أَرْمَلٌ → أَرْمَلَةٌ provide an important contrast?

Cause of Ghayr Munsarif: The Pattern of a Verb

• Because every adjective is ghayr munsarif
• Because it is a proper name
• Because its feminine uses round tā, so the taught adjective condition is not fulfilled
• Because it has additional alif-nun