When the Dumbledore sees himself on the Map in chapter 79, it initials his middle names:

When he was alone in the room, the old wizard looked down at the map, which had now written upon itself a fine line drawing of the Gryffindor dorms in which they stood, the small handwritten Albus P.W.B. Dumbledore the only name left therein.

But when it shows the Tom Riddles, it doesn't include the middle initial M.

This is probably just an oversight.