Sub Upgrade Question
 

I am on my 2nd career in SHIV. In the first career I started in the Tambor Class USS Thresher. I did 3 patrols and was given my Gato Class USS Drum. On my first patrol I was sunk by a JAP Plane. So I started my 2nd career in a Porpoise Class USS Pike. Again I did 3 patrols and was given my Gato Class. Again it was the USS Drum. Is this a pattern or a bug? or are you always supposed to get the Drum? What are the odds?