Check your NATS message. You'll see altitudes missing amongst the tracks. Typically the southern or possibly the northernmost track or two. That's for random routes that protrude into the regular tracks.
Southern U.S. to N. Europe or routings from the U.S. to will have random routes that sometimes touch one, or two, of the tracks.
Example - Altitudes 310, 320, 330, 350, 360, 380,
340, 370 and 390 would be used for random routes touching the track. Lower altitudes typically reserved for 767's or long haul 777/A330 flights and higher altitude for lighter 777/A330's.