What curious thing does David discover? Why does this intrigue him?

From the text:

One thing I discovered, which put me in some doubt. This was an entry on the fly-leaf of a chap-book (one of Patrick Walker’s) plainly written by my father’s hand and thus conceived: ‘To my brother Ebenezer on his fifth birthday’ Now, what puzzled me was this: That, as my father was of course the younger brother, he must either have made some strange error, or he must have written, before he was yet five, an excellent, clear manly hand of writing.