It may be just bad luck.
Just finished a patrol using 1.45 RUB and SH3Cmdr 2.3, during which I sunk 5 T3 tankers and a C2 merchant.
4 of the tankers were from a convoy that contained 5 T3s and at least 1 T2.
The other tanker was from another convoy which contained a T3 and 2 T2 tankers; could see more of that convoy due to better weather conditions.
Sometimes you get runs of bad luck; I recall one where in 39/40 every convoy had 5 escorts (pre V1.45).
Hope it helsp.
