The way that we perceive things is affected by past experience, and it is common for two people to look at the same event and interpret it differently. For this reason a serious mistake can often be made on a driving lesson or driving test, and the pupil that has made the mistake will not accept it, or in the case of a driving test, claim the examiner wasn't paying attention or didn't see something.

To prove that what you see isn't always what's happening, stare at the black cross in the centre of the circle for a few seconds.

Although this image is only an amusing effect, it isn't so amusing for a learner driver sitting at a busy roundabout trying to work out where all the traffic is going. On the other hand, your instructor or examiner has probably had years of experience driving both privately and professionally. Because of this past experience their perception at roundabouts will be extremely acute, and they will be able to read a complex situation and interpret it correctly with just a mere glance.