How are capture circles worked out for NPC port raids

[Archaos]

How have the circles position in the AI port raid battles been worked out as the positions for some ports make the ports harder to defend.

For example look at two ports with fairly similar coastline, Truxillo and Conil, both have a spit of land extending out even though Conil is a narrower bay. But look at the position of the circles for the AI port raid in the attached shots. 

I know which port I would rather try and defend. Why has Truxillo got a circle out on the other side of the headland and Conil does not? In Conil the square fort can engage before the AI gets close to the circle where at Truxillo the AI are practically in the circle before the fort can engage. The circles at Truxillo are all well away from the port while in Conil circle A is very close to the dock and the circles follow along the coast.