Because if you were to go over 4 on the x axis, and than go up 5 on the y axis, than you would be at the point (4,5) and than go over 10 on the x axis, than go up 13 on the y axis, which gets you to the point (10,13) and than lastly, you count the units (squares) in between the 2 points (from point (4,5) to point (10,13) which is 8.
