I think it is probably because most citizens of Southampton are good people, with respect for authority; ergo a greater proportion of crimes are reported.
Many of the citizens of some of our less salubrious sink holes, Portsmouth for example, suffer from a sort of gypsy-derivative inferiority/mistrust complex when it comes to dealing with emanations of the state such as the police (with the notable exception of the benefit office of course). As a result, huge swathes of nasty, violent acts go unreported.