As of the Disharmony expansion, I've noticed a discongruity between the fleets and fleet power I can see from the planet perspective and what appears once I hit the button to engage a fleet.
It used to be that you hit the attack button, and your selected fleet squared off against their strongest fleet. Now, it seems, it still selects that fleet, but sometimes there are additional ships attached - almost like the individual fleets from the system can squadron up to some extent. The fleets appear distinct afterwards, so this would break the rules as presented to the player, who cannot split fleets except by disbanding.
What will tell me, when I look at a set of enemy fleets in one of my systems, exactly the group of ships I'll face when I attack?