previews and menus vs actual movie resolution

[bsmooth]

This is really starting to bug me. I watch the previews and see the menus for the DVD's and Blu-rays,and they seem to almost be in a different resolution.
Most of the movies look great, once you get to them, but the previews often display lots of artifacts, and don't look the same as the movie itself.
Some of the menus look downright terrible.
Could someone explain why this is or someway to make the movie and previews/menus look the same?.