A little excess for strengthening in their case can only help, is probably what the instructor is thinking. Apart from the armor, there's also the Kngiht's weapons. Sword, shield, lance, what have you...whatever they're assigned. They're training them to be able to make their way as footmen from the sound of things, so they'll also be bringing their survival kits along with them. The totaal amount of weight will certainly go past the weight of just their armors.