Thanks for your replies!
@Leitender: I think people back then were at least as 'bureaucratic' as people today
As desertstriker has pointed out, at least in the Luftwaffe the confirmation rules were very bureaucratic.
But I agree that (in most cases) it's not really realistic that BdU knows your tonnage more accurately than you do (and could compare them). However, for the mod to work properly one has to encourage the player to report the truth as good as possible. To achieve this, the reports have to be checked somehow and the only available reference is the actual performance/sinkings. So one should regard the difference between your reported values and the actual values as a measure for your knowledge and the details in your reports.
I'm sure BdU and other officers had some feelings about the reports of Kaleuns and how reliable these were. And this is reflected in your 'renown', a quantity difficult to measure (if you play with renown 'on' you will see that BdU hardly ever changes your reported tonnage; only your renown is changed/modified reflecting what BdU thinks of your report).
Regards, LGN1