True ability from sailors is only one of a few factors that define accuracy.
It is important to remember that the maximum amount of accuracy a gunner can provide increases every time they level, so even once they reach the maximum accuracy for the level they are they will still continue to get more accurate but only when they level up, since that increases the maximum.
But, if you're struggling with those BB4 guns as far as accuray is concerned, you may need more experts/vets or a boost for your gunners. The maximum accuracy lvl 90 gunners can provide from their ability is around 1,500,000 so compare your gunners to that. If they are over it, your only option is to keep leveling (although your accuracy should be fine if that is the case). If not, then more expert/vets/boost and more leveling is needed.