Thats cause you can move the mouse more accurate with low sens, but its harder to make quick turns.
Also when you change sens it will take a while to get addapt to it since your muscle memmory has to get used to the new sens to.
-Degrees Per Inch :19.8
-Degrees Per Centimeter:7.8
-Inches Per 360:18.2
-Centimeters Per 360:46.2