Stamina decreases slowly over time, so it's quite possible that your player was at 7.012 and fell to 6.998 or something to that effect (my numbers should not suggest actual values, other than to illustrate sublevels of skill).
The reason for stamina decay is to force teams to train it every so often to keep their players in shape, rather than just training it once and having proficient stamina forever. Most teams use the break between seasons and the all-star break to train things like stamina and FT, as those weeks have only 2 games rather than 3.