Why are some movies only avaiable in SD on the apple tv, but on other devices the same movie is available in HD?

I have a 3rd generation apple tv. When I browse the itunes store on my apple tv, certain movies, such as singin' in the rain, only show as being in SD. However, when I browse the itunes store on my macbook, singin' in the rain is available in HD. Furthermore, if I buy the movie on my macbook and play it directly from my apple tv, will it play in HD?