It's the ratio of the match distance (i.e. X inches = Y degrees) converted to physical distance and normalized so the ratios are directly comparable on both sides of the equation. Technically the wording should be something like "The ratio of movement between the mouse and the crosshair to the selected monitor match distance of 75% is 1:21.5237". It's there to make it easier for people to see what value is actually being matched in the calculation, and if it's not perfectly matched how far off it is. But the wording can be confusing indeed, it's probably best changed to base length or something.