The explanation is evident, the verse is plainly genuine, and the Codexes which leave it out are corrupt.
One word about the evidence of the cursive copies on this occasion.
Tischendorf says that 'about forty-five' of them are without this precious verse of Scripture.

I venture to say that the learned critic would be puzzled to produce forty-five copies of the Gospels in which this verse has no place.

But in fact his very next statement (viz.

that about half of these are Lectionaries),--satisfactorily explains the matter.

Just so.