The list says 心 rather than 心脏, so maybe biological hearts are counted differently?

*checks* Hmm. MDBG says 心脏 can be counted with either 顆 or 个...

(I haven't read the Young Wizards books, but based on your comment I'm wondering if what you're referencing is something about "heart" as, like, "core" / "kernel" / "inmost center"? Because that would make sense!)