I take your point about that tiff originating as a jpg. Now that I think of it, I sometimes first save a photo in jpg as a tiff, and then edit that... if I quit, save and come back later, I don't lose quality as you do when you work on a jpg.
I did try selecting a few steps, increasing the contrast and then enlarging it. I first noticed that the continuous tone first row is not continuous at all... it consists of very narrow but discrete steps. And I think I spotted the halo effect at the edges of the 21 step row you mentioned.