For most people, predictability and consistency mean pretty much the same thing. Actually, I think there is a clear distinction: Consistent means something does the same thing each and every time; predictable means it does what you expect it to do. Let me give you a quick example.
In my house, all the electrical switches look alike and were approved by the same organization (Underwriter’s Laboratories—UL). That’s consistency. But when I travel somewhere I’ve never visited before, I expect to see switch-like objects next to doors that control the lights in a room—assuming there’s electricity. That’s predictability. The chances are really good these devices will either toggle in some manner or be a button that clicks on and off.1
As always, creating a proper shared reference lies at the heart of many predictability issues (see Chapter Seven). And retroductive inference also plays a huge role (see Chapters Eight and Nine—and stop skipping around).
1 Of course, there’s no guarantee that the switches are predictable as to either their function or location. See the “Tales from the Trenches” in Chapter Six. Or Google “Light switches - Mumbai, India” for a really funny article by “Steve.” Check out his other articles, too. He has a lot of great anecdotes about usability as it applies to service design.