Why does Enumerable.ToLookup<>() return an ILookup<,> and not a Lookup<,>?

There is one method in Lookup<,> that is not in ILookup<,>:

public IEnumerable<TResult> ApplyResultSelector<TResult>(
    Func<TKey, IEnumerable<TElement>, TResult> resultSelector);

Why is the return type of Enumerable.ToLookup<>() declared to be ILookup<,> despite the fact that it always seems to return an instance of Lookup<,>? If the return type were instead declared to be Lookup<,>, the above method could be used without a cast.