Because it is a mathematical model, and a reasonable one. When you have p( 211 to 220 ) = .0002 as in the article you don't need to consider negative values. Th

Because it is a mathematical model, and a reasonable one. When you have p( 211 to 220 ) = .0002 as in the article you don't need to consider negative values. The power of mathematical models comes from their ability to abstract these details.