Because Leopard will be completely resolution independent. Basically, you'd get the screen real estate of the high resolution (1900x1200) while the UI elements (icons, text, etc) will look like they do at a lower resolution (say the 15" 1440x900 or the previous 17" 1680x1050). Larger and more readable but with the benefits of being able to display a full 1080p video without stretching/scaling.
October can't come soon enough.