I've got it!!

[Alina]

No: HPFGUIDX 103265

Princess Di was actually a witch and Justin Finch-Fletchley is actually
Prince William! Thus we have Half Blood Prince in the book six title!
Mystery Solved!

We now return you to the regularly scheduled book discussions.

Alina.

P.S. yeah yeah, I know the Prince William is actually two years too young...
but Prince Harry is four years too young, so I went with the lesser of two
evils. Besides, it's just a joke.

*peers around and runs to hide from the strict on-topic list elves*