Difference between Select and Observable.Create

ObserveVsSelect.cs

void Start ()
{
    var onNext = UnityObservable.EveryUpdate<float>(observer =>
        {
        Debug.Log("on next call: " + Time.time);
        observer.OnNext(Time.time);
        });
    var select = UnityObservable.EveryUpdate<Unit>(observer => observer.OnNext(Unit.Default))
        .Select(_ =>
        {
            Debug.Log("select call: " + Time.time);
            return Time.time;
        });

    Task.Factory.StartNew(() =>
    {
        Thread.Sleep(5000);

        // Works perfectly fine since OnNext is called from the Unity thread.
        onNext.Take(3).Subscribe(t => Debug.Log("OnNext" + t));

        // Fails because select is called from a different thread than the Unity thread.
        select.Take(3).Subscribe(t => Debug.Log("Select" + t));
    });
}