Thats largely to do with the differences between a TV CRT tube (and its inability to display single pixels accurately) and an LCD though, its not the same as the difference between HD and SD. With plasma you get sharp pixel definition, pretty much the same as LCD.
For a more comparative view set your 360 to 868 x 480 and display it through your VGA monitor. That still wont be a true comparison though if you are using LCD as it wont be in native resolution so there will be some scaling going on, but it will giver a better indication than trying to compare against a TV screen.