It's counter intuitive, but it has been indicated that these numbers DO NOT reflect sunk ships (or at least not only)
1) This is a cargo tonnage equivalent (if I remember well, 250 tons per "ship", which is small for an actual cargo)
2) This represents unavailable tonnage for a number of reasons : sunk ships because of U-Boot, yes, but also ships sunk because of bad weather (or other maritime related hazards), and SHIPS UNDER MAINTENANCE ! And every once in a while, every ship has to go to port for a couple of months to scrap the hull, repair the machinery, let the sailors go to their family / tavern... which will count as a "loss" in WitW.
=> Bottom line : you are bound to "lose" a whole lotta "ships", but this is not critical.
EDIT : the increase in losses is directly linked to the level of usage of the ships. So in the turns just after D-Day, there is a huge level of maritime activity, which will result in higher losses (but not necessarily higher loss % compared to usage, though this could be checked)
< Message edited by EddyBear81 -- 4/10/2018 6:54:23 AM >