Not the usual Fuel Question

[GunnersMate]

OK I'm on a patrol in Celebes when I spot a medium convoy. I take out a couple freighters and a destroyer on my egress. Second time around I get greedy and caught a D/C square on. I was able to get that SOB with a MK27 . (love thos cuties ) Anyways I was able to limp back to the tender just east of Celebes with about 68 Hull damage and a destroyed battery, radio, periscopes, and a diesel. I noticed right afterwards my diesel went to about 30-35 so I figured a bunker tank was venting. No problem I can get it fixed. After leaving said tender (and I got a lot of not so tender things to say to those engs ) I run into a small H/K group about 20 miles out. I've seen about a dozen of these thing and they always consist of a gunboat and a subchaser and I thought Lunch Time! It was actually a gunboat and an Akizuki I hightail it out of there but its to late and the destroyers coming my way. I go PD and try a stern shot but he's zigging and my torps were zagging. The cutie goes after the gunboat and connects (unless he ran into a regular torp but that would have been 1:1000000 and luck isn't my strong suit this patrol ) After an hour or two of cat and mouse he hasn't hit me nor I him when a torp connects and hits his props or stearage. He doesnt prosecute just tries to run so I put another torp in his aft (out of a 3 salvo) It doesn't do squat but convince him to cut his losses and try to head home. At about 1700 yds I launch a 4 shot spread of which two connect and he's just obliterated. After expending half of my torpedo rooms on 2900 very difficult tons I go up the road to fill up After leaving I notice my fuel's back down to 30-35 and I still have to complete my patrol. Can anyone tell if this is a "feature" a bug or a one time glitch? I do enough of watching the fuel guage in my car I dont want to do it in a game.