Stamina, like all skills, has sublevels. So average stamina, for example, might be anything from 6.00 to 6.99 stamina. Unlike other skills, stamina gradually degrades every week for all players, regardless of age. So your rookie started with a very low sublevel (e.g. 6.01 if he was average) and then dropped to a very high sublevel of the tier below (e.g. 5.98).
You can reduce the amount of stamina decrease through a Trainer with the Fitness specialty and/or a Nutritionist. With very high levels of Nutritionist, stamina decay will completely stop or you might possibly get _very small_ increases of stamina on a weekly basis (estimates for a Level 7 Nutritionist range around 1 pop every 2-3 seasons)