Optical effect

When I look at the spinning wheels of a tractor trailor in my rear view mirror I can see a strobe like effect, i.e. the wheel lug nuts seem to stand still or slowley rotate much like looking at a rotating object under a strobe light. However, when I look directly at them at the same speed and conditions, I do not see this effect. This happens during broad daylight so there is no influence from artificial light. Is there an explanation for this?