Why Was There A Gleam of Triumph in Dubledore's Eye At the End of Book 4?

This has been bugging me for years.

I know it's important.  Jo has said it's important. When asked about it, she said, "Good question... excellent question, in fact, and like all the best questions I get asked, I can't answer it! Because it would give too much away. However, well-spotted. Have fun guessing... someone's bound to get it right!"

But what did it mean?

It seems as though Dumbledore knows he has fooled Voldemort somehow. Like maybe Voldemort thinks he's defeated all of Harry's protections, but Dumbledore knows better. But surely Dumbledore knows that Voldemort knows that wasn't Harry's only defense. Lily's blood kept Voldemort from touching Harry, but it also protects Harry while he's at the Dursley's. There's no evidence that protection was taken away in the graveyard at the end of Goblet of Fire.

Or maybe it has something to do with the whole Harry-is-a-horcrux theory.... But it seems that if Harry is a horcurx, and Dumbledore knew it, he would have told Harry so that they could have collectively worked on a way to remedy the problem.

I go round and round, but I just can't figure out what the was all about.