Doug Fattic 

Yes, heating on one side curves the tube in that direction. When it is massively heated enough to flow all that brass (it is actually bronze that melts at 1600º) around one side of that seat tube, it will curve a lot in that direction. The cause of the bend on a lady's fillet brazed Schwinn Super Sport seat tube would not come up for discussion on a frame builders forum unless it was a newbie because the cause would be obvious to anyone with brazing experience.

Heat and its resulting distortion is one of the complications of making a frame that is correctly aligned. It isn't easy to do and many vintage steel frames are not properly aligned. When thin wall/heat treated Reynolds 753 tubing came out in the late 70's, they required a test frame to be sent to them before a builder could purchase it for building themselves. They checked it for alignment and cut it apart to check brazing penetration. In talking to the Reynolds representative back then he told me all Americans that had applied up to that point had failed their test. The problem is that heat treated tubing can not be bent (well not much anyway). Old Reynolds 531 was designed so that after brazing, the frame could be bent into alignment.