I'm guessing something about the production process made it easier to fit things together this way. I was just reading a Roadmaster ad from '52 that bragged of a "100% electronically welded frame" whatever that means.
I just went and had another look, all the tubes going to the bottom bracket were done like the top tube. The rear fender bridges and seatstays at the seat cluster are done like the middle tube. Maybe the main triangle was done in a machine and the rest was still done by hand at this time.
I stole that basket off my wife's bike 3 weeks ago and she hasn't noticed yet. She never rides that thing. I've been running errands on mine.