Imagine you're holding a ball with drawings on it. Hold it out at arms length and fix how it looks in your memory. Now bring it close to your face and move your head a tiny bit to the side. You're not seeing the whole back-side of the ball. Far from it! However, you are seeing some bits you weren't seeing before and the whole picture you can see now looks different than it did when the ball was at arm's length.

That's my guess. They're seeing parts of the dark-side of the moon because they're now close enough that they have a different viewing angle than we do on Earth. Remember, they're not flying straight at the moon. That's not how transfer orbits work.