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.
Every able-bodied student is capable of breaking one one-inch pine board with a Taekwondo side kick. Many are able to do it consistently during practice and are able to break even greater thicknesses. Yet, under the stress of testing, many students cannot break one board, even after multiple attempts. Sometimes the students who fail to break the board are large men in excellent physical condition who are capable of performing difficult jump-spin side kicks in class, but under stress, they hit the board off-center or miss it entirely.