Why does عَصًا become عَصَوَانِ in the dual?

Dual and Plural of Mamdud and Maqsur Nouns

• It is a three-letter maqsur noun whose original waw returns.
• Every maqsur noun changes its final alif to hamzah.
• Its feminine hamzah changes to waw.
• The sound masculine plural ending is attached.