Not that uncommon for the bike boom era. Manufacturers saved a lot of money on their lower-end models by building up an inventory of top tubes prebrazed to seat lugs and upper head lugs and down tubes prebrazed to bottom brackets and lower head lugs. That way, they needed only one length of top tubes, one length of down tubes, and one set of lugs for all sizes of their frames--no need for lugs in all the different possible angles--and they'd braze in longer seat tubes and head tubes for big frames and shorter tubes for small frames. Fast and cheap. This is the dark side of the classic-and-vintage era; they weren't all artisan builders obsessed with perfecting their craft.