Seat maps that don't map exactly to the aircraft type normally mean that they are keeping their options open for a change to a different aircraft or config (in this case, probably a 737-800).

If they don't make the change, the extra seats will appear sometime before departure (often 72 hours, but it varies). If they do, the aircraft type will change to (say) a 737-800, and nobody has to have their seats reassigned as those in row 5 were never allocated.