Gravity Analog TDS Sensor Coefficients

I am a student currently developing a water quality sensor for a project. I have purchased your TDS and temperature sensor to get a general idea on how it works. Everything turned out to work perfectly fine, as I used a more expensive TDS sensor as a reference point and the output values were quite similar. When looking into the code however, I saw that once the analog reading was converted into a voltage value, to convert the value into Electrical Conductivity, you used the following formula:  

(133.42V^3 – 255.86V^2 + 857.39V)*KValue

I am very curious as to how you guys obtained these coefficients as I have looked every where and cannot find the reasoning. Can you tell me how you got these coefficients for this cubic polynomial? 

Thank you and kind regards