in thickness, it is far easier to weld to the inside of the flange where it is next to impossible to "burn" a hole through the thinner tubing since it's fully supported by the flange. I believe this practice lends itself to a higher speed of manufacturing with less capable operators... = cheaper to produce.
IMO I would weld everything both inside and out and stress relieve the parts in an oven when done. Then machine or grind the gasket surfaces.