Then place the beam between two high buildings. Now ask the same people if they think they would be able to walk down the length of the beam without falling off. Most people would say NO! When asked to try, only a very few would try and surely a few of them would fall.
Nothing physical changed in either of the scenarios. The width or length of the beam did not change; the only variable is the mental aspect. What was easy to do under safe conditions became difficult to do under dangerous conditions. What is easy to do under normal conditions becomes much more difficult to do under stressful conditions, even when all other factors are kept constant.
If is no more physically demanding to perform a pattern at a rank testing as it is to perform the pattern in a training class. , when under the mental stress of testing, many students freeze, lose their place, add movements, leave out movements, put movements in the wrong order, or perform sloppy techniques while performing their patterns.