I felt the same way about "ordering off menu" and "ordering off the menu" (the former, you're asking the chef for something special; the later, you're making a choice from what's printed). It's even worse when people announce their availability with "out of office". The idiom seems to be based on intimate knowledge of how the person normally works, where they expect to be, and how much of the sentence is implied ("I'll be out..." vs "I'll be working..."). Idioms are funny that way.