WHAT DO THE FONT STACK PERCENTAGES MEAN?
With CSS, if a single person happens to have the first named font installed, the chance that any other font is displayed on their screen will be zero. When you apply this principle to a large number of people like Web users, it is possible to calculate an overall probability that each font in a font stack will be displayed because some people will not have the first font, second or third fallback font. These percentage probability figures therefore equate to the proportion of people whose browsers will display each font in the stack, depending on which operating system they use.
A CSS font stack is a priority list where the first matched font is selected, so the overall probability that fonts at the top of the stack are installed and displayed affects the proportion of people whose browsers will display those further down the stack. If the sum probability for all the named fonts is less than 100%, what remains is the probability that an un-specified generic font family will be displayed. In other words, a proportion of users may not match any named fonts and their font selection will fall through to the generic font family.