Surprising results setting the black point in a Curves adjustment layer

I have the following working spaces in my Photoshop Color Settings:

RGB: Adobe RGB

Gray: Gray Gamma 2.2

I open the following Photoshop document, which is in Adobe RGB

The image has six square cells with HSB values (0,0,100), (0,0,99), (0,0,98), (0,0,97), (0,0,96), and (0,0,95). I included the marching ants in the screenshot so that you can see the pure white cell on the left.

I add a Curves adjustment layer and set the black point (using the black eyedropper) on the rightmost cell.

I expected this to change the HSB values to 

(0,0,100), (0,0,80), (0,0,60), (0,0,40), (0,0,20), and (0,0,0)

but it did not.  The actual values, measured with the eyedropper, are

(0,0,100), (0,0,80), (0,0,61), (0,0,41), (0,0,22), and (0,0,2)

OK, so maybe there's a numerical issue, since HSB is not a real color model and Photoshop is working in its own internal numbering system.  I thought I should be able to fix it by using the "click and drag" feature in the curve adjustment. But when I tried to click and drag in the (0,0,2) cell, Photoshop would not let me lower the value below (0,0,2). It let me raise it, but not lower it.  This is really puzzling, and I hope someone can explain why.

One possible solution is to (1) fill the last cell with solid black and then (2) "click and drag" to adjust the tone of the other cells.  However this would defeat the purpose of the exercise, which was for this to happen automatically when I originally set the black point.

I have two questions. (1) Why won't Photoshop allow me do click and drag the rightmost cell down to (0,0,0) and (2) where else is my thinking wrong? 

EDIT: If I work in RGB, setting the black point does the right thing. For example, if I have eighteen cells spaced one unit apart from 255 down to 238, and I set the black point on 238, I get 

255, 240, 225, ..., 15, 0    (equal steps of 15).

This only works for numbers that evenly divide 255: 1, 3, 5, 15, 17, 51, 85, and 255 itself.