Why does F#'s Seq.windowed return seq of array

Seq.windowed in F# returns a sequence where each window within is an array. Is there a reason why each window is returned as an array (a very concrete type) as opposed to say, another sequence or IList<'T>? An IList<'T>, for example, would be sufficient if the purpose was to communicate that the items of the window can be randomly accessed but an array says two things: elements are mutable and randomly accessible. If you can rationalise the choice of array, how is windowed different from Seq.groupBy? Why does that latter (or operators in the same vein) not also return the members of a group as an array?

I'm wondering if this is simply a design oversight or is there a deeper, contractual reason for an array?