I have an air patrol over a fleet. In this screen, a german fighter attack my fleet. Why my fighter doesn't defend my fleet ?

however, I read this in the manual :
7.1.9 COMBAT AIR PATROL
Players may move Air units over a... a sea region free of enemy units as long as it spends no more than one half its current movement total to do so (air units so moved must be able to return to their home base region, ergo the limitation of one half its movement range). By doing this, the player is setting up a 'combat air patrol' whereby any enemy unit entering that region will be forced to engage in combat.
TACTICS TIP: It is often a very good strategy to use patrolling aircraft to fly protection over friendly fleets, (page 60 manual pdf )
Thanks !