Because the 'play perfect' neurons have a right to be on holidays
Ok, to be serious. There is some sense in the joke. Humans are no robots that are always in the same shape and always react the same way. Your brain can be tired, occupied with sth. else (maybe subconsciously), does not get enough oxygen because of flat breathing due to nervousness, etc. Your muscles can be tired as well or just conditioned for sth. else you did for a while