Why does Allah Almighty say "فيهما" (in them - dual) regarding the two Gardens sometimes, and "فيهن" (in them - plural) at other times?

The two Gardens are mentioned in the dual form in His Almighty's saying: (But for him who feared the standing before his Lord, there will be two Gardens) [Ar-Rahman: 46]. Then two other Gardens are mentioned in His saying: (And below these two are two other Gardens) [Ar-Rahman: 62]. Then the plural form (In them are good and beautiful ones) [Ar-Rahman: 70] is used to refer back to the four aforementioned Gardens.