A mistake I noticed in the Stephen Fry audiobook of OOTP

In Chapter 23 "Christmas on The Closed Ward", the book reads as follows:

"Dobby did, he left the Malfoys' to give me warnings three years ago"

At 27:00 into this chapter in the Stephen Fry audiobook he says "Dobby did, he left the Malfoys' to give me warnings two years ago".

This struck me as strange listening to it since I knew immediately that two years ago was incorrect. So I went back to my book and sure enough, it said three.

Forgive me if this has been posted, but I searched and could not find this. Interesting little mistake there in the audiobook.