In game 437974, I had lost Caesar in a melee and there were around 12 prisoners on board - these are the remnants of my crew.
When I recaptured the Caesar on turn 21, the melee report stated that I had captured 3 prisoners from the prize crew.
BUT, when looking at the ship, I find my old crew are still prisoners along with the 3 I just captured!
So now I have 15 prisoners instead of a large crew!!!