The reasoning behind the 720x540 is that NTSC does not have square pixels (they have a 0.9 pixel aspect ratio) but your computer monitor does have perfectly square pixels. If the viewport were actually shown as 720x486 then the aspect ratio of everything you are looking at in the viewport would appear squished. Luckily HD standards have solved the problem by making all pixels perfectly square in all HD resolutions.
You can adjust the title safe markers in the scene if you need to to match your output. The option is on the Scene tab in the Object Inspector, or you can set default values when creating the project.
#XPression