Not sure if this applies to Arabic, but sometimes this happens when there is a old case which got lost and which had the same suffix as feminine singular. Usually that's genitive. For example Croatian (I'm a native speaker) uses genitive plural agreement for number >=5.

Feminine genitive plural happens to have the same suffix as feminine singular, and it could be possible to envision an future evolution of the language where the genitive is lost, while the number agreement rule could be preserve. So it's possible that grammars of this future language would state that feminine plural agreement for numbers >=5 you have to use the feminine singular.

Btw, I once read a very nice title of a german language book:

"Der Dativ is dem Genitiv sein Tot"

literaly, "the dative is [to] the genitive its death"

Which denotes the fact that in modern German the genitive case is being lost in faviour of constructs based on dative (the correct form would be "Der Dativ ist der Tod des Genitivs", i.e. "the dative is the death of the genitive")

(http://en.wikipedia.org/wiki/Der_Dativ_ist_dem_Genitiv_sein_...)