To reproduce:

Color[] colors = new Color[]
{
    Color.LightBlue, Color.LightGreen, Color.LightYellow,
    Color.Red, Color.Orange, Color.Pink, Color.Purple, Color.Peru, Color.PowderBlue
};
Random rand = new Random();
for (int i = 0; i < 25; i++)
{
    Resource resource = new Resource();
    resource.Id = new EventId(i);
    resource.Name = i + ".Resource";
    resource.Color = colors[rand.Next(0, colors.Length)];
    this.radScheduler1.Resources.Add(resource);
}

this.radScheduler1.GroupType = GroupType.Resource;
this.radScheduler1.ActiveView.ResourcesPerView = this.radScheduler1.Resources.Count;

for (int i = 0; i < 3; i++)
{
    Appointment a = new Appointment(DateTime.Now.AddHours(i), TimeSpan.FromMinutes(30), "A" + i);
    a.ResourceId = this.radScheduler1.Resources.Last().Id;
    this.radScheduler1.Appointments.Add(a);
}

NOTE: it is also valid for the horizontal scrollbar in Timeline view.

Workaround: use the SetResourceSize to increase the last resource's width a little bit: http://docs.telerik.com/devtools/winforms/scheduler/views/grouping-by-resources