First q: theoretical amounts determined by stoichiometry assume there is a complete reaction i.e. all your ammonia and oxygen becomes nitric oxide and water. However, in an equilibrium reaction it never goes to completion. As such this means that you can never have 100% yield or the theoretical amount. It's just a consequence of some of your products in equilibrium ALWAYS decomposing back to reactants
Second part: is a simple q on le Chatelier's principle and how temperature affects equilibrium position
The reaction you have is exothermic.
Increasing temperature disturbs an equilibrium. According to Le Chatelier's principle when a system at equilibrium is disturbed it undergoes a chemical reaction to minimise the disturbance. Increasing temperature shifts the reaction to the endothermic side, which is the left. this increases the rate of the reverse reaction and decreases rate of forward reaction. therefore, concentration of nh3 and o2 increases, while concentration of no and water decreases. Therefore, explaining why less NO is made