text: Here's a comparitive map of the city area of Plymouth which has a similar population to Hull, showing the city node definitely not at the center of the city area:
  http://www.openstreetmap.org/relation/189924#map=12/50.3721/-4.1370&layers=T
  Maybe Plymouth has a more definite center?  The city node is hugely important in e.g. planning which train station to get off at, or which hotel to book when you know nothing else about the city.