The lines have nothing to do with design, they were made by the cutting tool that cut the tenon. Either the tenon was cut too fast making the ridges or there was slop in the tool post or the piece that held the cutter. I have a very old tenon cutter and it does the same, the newer ones don't as much. And if they're cut on a lathe properly they'll be very smooth.
Just the nature of the beast as it was.
Mine is very similar to this new one but without the ring adjustment and my cutter is one piece, not to be confused with the cheapy PIMO's.