Which of the two scales is the "right one" on the antifreeze tester?

I’ve bought an antifreeze tester yesterday, and the scale on the floater is divided as follows:

• the upper half is for methanol-based antifreeze, the lower half is for the ethylene glycol-based one (so far it is clear which one I have to look for, since my anti-freeze is ethylene glycol-based)

• however both halves are divided again into two separate sections (the upper one is green, the lower one is white), and the numbering is not continuous, but restarts from low negative in the white sections. (EDIT: and it covers again the same range as the green one!)

See illustration and picture at the bottom to make it more clear.

Question: how do I know if I have to check for the upper, or lower section of the ethylene-glycol scale? (Also, implicit question: why are there two different scales at all for one kind of material?)

Remark: I tried to measure my (ethylene-glycol-based) antifreeze, and it showed -17 degrees in the white section, but I don’t know if the measurement was “right” or not.

Illustration

    +--+
    |  |
    |  |
    |  |
    +--+ -40 degrees C    ---+
    |  |                     |
    +--+ -34 degrees C       |
    |  |                     |
    +--+ -29 degrees C       |
    |  |                     |  green
    +--+ -23 degrees C       |
    |  |                     |
    +--+ -17 degrees C       |
    |  |                     |
    +--+ -12 degrees C    ---+
    +--+ -34 degrees C    ---+
    |  |                     |
    +--+ -29 degrees C       |
    |  |                     |
    +--+ -23 degrees C       |
    |  |                     |  white
    +--+ -17 degrees C       |
    |  |                     |
    +--+ -12 degrees C    ---+ ------------------ METHANOL
    +--+ -12 degrees C    ---+
    |  |                     |
    +--+ -17 degrees C       |
    |  |                     |
    +--+ -23 degrees C       |
    |  |                     |  green
    +--+ -29 degrees C       |
    |  |                     |
    +--+ -34 degrees C    ---+
    +--+ -12 degrees C    ---+
    |  |                     |
    +--+ -17 degrees C       |
    |  |                     |
    +--+ -23 degrees C       |
    |  |                     |  white
    +--+ -29 degrees C       |
    |  |                     |
    +--+ -34 degrees C    ---+ ------------------ ETH GLYCOL
   /    
  |      |
  |      |
  |      |
  |      |
  |      |
  |******|
  |******|
  |******|
  ______/

Picture