5 ships sunk with 4 torpedoes

[Tupolev]

So I had an interesting experience on my last patrol. I'm running NYGM

May 1941, Grid AL29. Spotted an inbound convoy just after dawn. Due to the length of the day, I decided to attempt a daylight submerged attack with hopes of being able to surface and run ahead later for a follow-up night attack.

I successfully closed for an attack and fired 4 singly aimed torpedoes at 2 tankers (1 large & 1 medium) and 2 merchants (2 large). I stayed at periscope depth long enough to confirm hits on each target, and then submerged deeper to make my escape. Two hours later, I surfaced behind the convoy.

One of the merchants had sunk already, the other was making about 1kt, down by the bow. Both tankers were down by the stern and stationary. I'm thinking, Great, easy pickings.

Then I noticed waayyyy off in the distance, a huge smoke plume. My curiosity gets the better of me, and I dash off towards it to check it out. Just before I get it fully in sight, I get a "She's going down!" message. No more smoke plume. The log shows I just sunk a Small Tanker. The Large & Medium Tankers are still afloat as well as the Large Merchant. After coup-de-graces on the damaged ships, my log now shows (in order):

Large Merchant Sunk
Small Tanker Sunk
Large Merchant Sunk
Medium Tanker Sunk
Large Tanker Sunk

So, now quite at length, my question to my fellow SubSimmers: How do I sink 5 ships out of a convoy when I only attacked 4 of them?

T