There is a somewhat confusing interaction between the histogram profile, the display profile and the colour picker. If you want to see linear values, set everything to linear. Setting the display profile to a linear space will produce the wrong screen output, of course, but you can do that if you want to study the algorithms of darktable. Just don’t forget to turn it back to your display’s profile; otherwise your subsequent edits will look wrong to everyone (been there, done that…).
If you open the image in the Gimp, it’ll give you readings:
0, 28, 56, 85, 113, 141, 170, 198, 226, 255.
Those are in non-linear sRGB space. You always have increments of 255/9 = 28 1/3 (so 28 or 29, for every 3rd step).
Set Gimp to use linear light. With 8-bit values (not well suited for linear light, but let’s use that now for the sake of comparison with darktable’s colour picker), the readings are
0, 3, 10, 23, 42, 68, 103, 144, 194, 255.
You can clearly see that the steps are not 1 EV apart (that would mean doubling the value in linear representation).
Darktable’s picker gives you the same (linear) readings (you need to set the histogram profile to a linear space)
Adding 1 EV:
You can see that the numbers doubled, up to 255, which is the maximum. Set your display profile to linear rec 709 or 2020, and the picker becomes unbounded, with values:
0, 6, 20, 46, 84, 136, 205, 288, 388, 510.