When you attack a fleet, or the ships responds themselves by stance, what usually happens is that they gang up according to the overload in preferences. 3-1 is default. I think what is seen is that often ships don't attack closest target, and may approach their target without firing at what is nearby. I am not 100% sure, though, I don't actually study the goings that closely.
When there is a large battle, I am usually controlling the fleet myself. Why? I like to focus fire on dangerous targets to get them down. I want to kill troop transports when they attack my colony. If I attack their colony I want all guns at a single defensive base/space port to disable a tough enemy. Once things are in control I stop giving orders, apart from maybe selecting ships and make them help a poor carrier that has 3 ships pounding it or something like that.
In fleet vs single ships I often also give all attack order, because the 3 chasers are nowhere close, and I get annoyed by watching 20 other ships sit by and watch with the offender running between them... I do micromanage my fleets until the point when far too much happens at once. After war I often find some destroyer raiding fleets short a few members after they have been forgotten during the general fighting chaos (I send them at mining bases or suspected mining systems with "engage system targets" and hope they come out alive, and little else does).
But during combat, a bit help in forcing focus fire and activity helps a lot. They are too lazy to all open up at once otherwise.