When you put some units on a ship, and then your enemy destroys it, in the statistics at the end of the game the units won't count as lost ones.
How to replicate:
1) Start a map with water on it (Island map are the best for it).
2) Build a ship
3) Load units on it
4) Go to your enemy
5) Let your enemy destroy your ship
6) End the game
7) In the statistics you will see a lost boat, but no lost human units