I need to understand the SHCommander Random tonnage system

What the Random Tonnage in SHCommander doing exactly?

I went back to my patrol (April 1944) and all my Tankers were about 11k Tons.

If I check back my previous patrols, they are different, varying from 8k to 10k.

So, is this working on different times of year? Or I was lucky to bag T3s all of this weights?

-----------------------------------------------------------------------------------

Another note,

I also noticed that the C3 draft has increased of 3 meters. Then revert to 9 meters. WTH