LaunchServices keeps track of which applications have been opened and which applications should open which documents. I’ve found that this database, which is sprawled in several locations on your drive, can become bloated or even corrupted.
Fortunately, Cocktail makes this easy to reset to the default state, which I suggest you do if you are seeing the same application in your Get Info “Open With” dialog or can’t change the application to open a file with.
Just launch Cocktail, click System, then Misc and rebuild the Launch Services database.