Often the difference between the perception of the test between the rider and the judge is an understanding of the purpose of the test, for many riders the test is something to get through rather than to consider what the demands of the test can tell the rider about the progress of the training of the horse.
Often, in my experience, riders don't look at a test with the benefit of the comments and directions shown on a test sheet that a judge works from
So riders make mistakes in terms of when they ask for transitions. A knowledgeable rider will make sure that any mistakes are limited to one movement and not risk losing marks in more than one movement.
In addition, if the rider has studied the directions on a test sheet then they will be thinking about the tasks that need to be achieved to produce good marks such as rhythm, quality of pace, bend and suppleness, balance and activity in hind legs rather than simply the goal of achieving a transition from one pace to another on a circle.
So make sure that, when learning your dressage test you don't only understand the 'what' but also the 'why'.
Kim Ratcliffe, List 1 Judge