All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Specifying the number of the poor to feed in every expiation is a devotional matter; the wisdom behind which wasn`t clarified by any of the texts of Sharia. Moreover, a Muslim should abide by them as they were narrated and believe that Allah, Tost Exalted, is full of knowledge and wisdom regarding His revealed religion. He Said (what means): "A. L. R. (This is) a Book, with verses basic or fundamental (of established meaning), further explained in detail,- from One Who is Wise and Well-acquainted (with all things)." [Hu'd/1]. Also, He Said (what means):" Thus does God make clear the Signs to you: for God is full of knowledge and wisdom." [An-Nu'r/58]. And Allah Knows Best.