Is the result of your training session random, or related to something the user controls?
According to serebii.net, "There are also three different tiers of results for each training regime, each giving higher Jump Power values of double or triple the base value, but this is selected at random so you have little control."
I can attest to this. Sometimes I can get 2 "Amazing" ratings in a row and then not get them again for a while. The rating system is likely based on a percent chance per tier but there is no information on what the chance of each tier is.