I assume you're talking about what it reads, when you hover your mouse over an enemy ship?
If so, then this is what your crew/spotter _thinks_ the status of the enemy ship is.
So your crew thought the 2nd enemy B was sinking but, in fact, it wasn't and after you left battle the enemy crew patched it up enough to reach a friendly port.
Yes, it sucks, but that's life
Note:
There is a way to tell, when an enemy ship is _really_ sinking.
Activate the "show ship in lower-left corner" thingy (somewhere in the options)
Hover your mouse over the ship, so it is shown in that picture
Right-Click that picture.
If the speed of the ship reads 3 knots, it's dead (this is different than clicking the actual ship on the map, which will _always_ show the design speed of the ship)
Edit:
Ok, totally mis-read this.
When the game tells you that the enemy has no ship left, then obviously all enemy ships are considered sunk.
This looks like a bug.