Because your ball never hit exactly x=280.
If you have a speed of 240 in your bullet-behavior, the ball will travel 4 pixel each tick, if you set the ball in the beginning to x=1 every tick it will travel 4 pixel, so after the first tick it is x=5 then x= 9 and at least x=277 and x=281
so it will never trigger x=280.
Even if you set it to x=0 a little slowdown could alter delta time and you will miss x=280 again.
Better is to use x>280.