Hi Tim,
the small neat welds on the bike tank are tack welds, to hold its 'structure' before the final welded seam.
If the blade was indeed arc/gas welded, it could easily be done in small increments leaving the blade to cool 'between' welds. This would limit heat transference (by convection) to the surrounding blade. The heat with welding tends to be 'localised' to the immediate area (dependent on duration of welding, thickness of metal, etc)
Also I have found a technique whereby the 'hot' weld is hammered to limit welding distortion.
If this technique was used on this blade ...it could explain the confusion.
"....Welding Distortion
Welding Distortion is caused by the localised heat that welding creates along with shrinking of the weld metal as the weld cools. It is difficult to avoid welding distortion but a few simple steps will help minimise the problem. If possible plan the job to avoid long welding runs. If long welding runs are unavoidable use plenty of tacks, weld with the welder turned as low as possible and do a series of short welding runs, allowing the workpiece to cool between welding runs. Using backing plates to help take welding heat away will also help.
Welding Distortion can be corrected from butt and outside corner welds by peening.
Do this by holding a metal block behind the weld and peening evenly along the weld with a hammer. This process stretches the weld metal to correct the shrinking that occurred during cooling. Avoid over peening as this will overstretch the weld and re-distort the workpiece....."
http://www.thewelderswarehouse.com/welding.html
Kind Regards David