well this means that he drops in stamina when his sublevel falls from X.1 skill to (X-1).9 skill.
u can understand better by this example.BB just shows us the major skill and not the sublevel.so if ur players has a stamina of 8.5 or 8.2 BB will show that his stamina is 8(strong).since we cant see the sublevels so we dont know what is the sublevel of that skill for that player.so he will face a skill drop in stamina from 8 to 7 this would mean that his stamina dropped from 8.1 to 7.9.
And one more thing,stamina drops regularly for every player so even when he has had a drop in his skill still his stamina would have dropped as much as it would have dropped a week before.its just that u couldnt see the sublevel dropping and so u didnt see the skill drop in sublevel as well.