True Aspect Ratio of LCD Monitors and Plasmas can be a very tricky situation. Most commonly Data or Monitor that will connect to a PC/Laptop are 16:10. Video or DVD/VHS style connectioned LCDS are usually 16:9 but some times can be different I.E. 15:9. By going into the controls of the monitor you can turn off the Zoom (wording varies by monitor) feature that will show you "True" aspect ratios. This will sometime show black bars on the sides, top and bottom, or all the way around.
There is a formula (I will type it up soon) to tell if a size of a monitor equates to be 16:9, 16:10, etc.