This ties into how ranging works -- most likely the shields of the bomber are high enough that even though the bomber is inside the range of the turret, the turret has a 0% chance of hitting it.
Basically, just because an enemy ship is inside the Z circle does not guarantee it can actually be hit. If the enemy has significant shielding, then that reduces the effective size of the Z circle (which isn't shown, because that varies by target). And if something is at the outside range of the Z circle, there's often a very poor chance of actually hitting the target. When you have one of your ships selected, you can hover over an enemy ship (in this case the bomber), to see what the percent chance of hitting it is. Given that Bomber Is have 1500 shields, it's almost a certainty that the shields make the bomber outside the effective range of that turret.
If you'd like more information on how shielding works here's a link to the mini faq:
Do Shots Miss Sometimes?Hope that helps!