It's an odd way of expressing it, but the scale seems to be the ratio of the values as measured between the top of the pot and the wiper (R9), and the bottom of the pot and the wiper (R10).
R9 + R10 must always equal 50kΩ so when the pot is at its midpoint, R9 = 25kΩ, R10 = 25kΩ so the ratio is 1.
If the pot is turned so that R9 = 10kΩ and R10 = 40kΩ then the ratio is 0.25.
Similarly, if R9 = 40kΩ and R10 = 10kΩ the ratio becomes 4. The ratio will go from zero to infinity.
I think that's what they're trying to say.