No, you're reading it right. Another anomaly, Mandela effect, timeline shift, whatever. Or maybe the BB was just struck with an SN in '82 and sat on a shelf for a year. The above-mentioned differences, at the very least the under-the-BB shift cable routing, lean toward an '83. I have a similarly mysterious 400 series, early '82 serial number (10920?), but clearly sporting the same cable routing as yours. Interestingly, and perhaps NOT coincidentally, mine also has a Tim Isaac BB shell (60TSI in my case). Maybe a number of his shells / lugsets were set aside for later builds.
Hmmm, trivia question that MIGHT shed a twinkle of light, for any "real" '83 owners following. Do the '83s that have an actual '83 SN do the cable routing with grooves and bolts like shown above? Or do they use the more commonly found plastic guide screwed into the shell? My mystery bike is cabled like the OP's; maybe that's a characteristic of the TSI shells :shrug: .