App variations i.e. "Safari" and "Safari.app"

On my MacBook I have had occasions where when I go to my list off applications sometimes it will list the apps with just their simple names i.e. "safari, mail, contacts, etc." and other times all of a sudden they will all have a .app listed after them i.e. "safari.app, mail.app, contacts.app, etc." Does anyone else have this happen and if so what is with the variation and why does it not stay just one way?