Anyone else find this happens?

[storm-petrel]

I seem to be able to sink a C3 cargo with only one torpedo, but the smaller C2 needs a good pounding with the deck gun after to sink the bloody thing. Same range and does not seem to matter what torpedo I use the C2 nearly always carries on sailing.
Just find it strange that the smaller cargo can take more damage than the larger one.
Using stock SH3 and no manual targeting ( as I am carp at this bit ) so just thought to ask the question.
Thanks yall.