i had this on an old phone (my home number was one digit from a friends) i think it's to compensate for international format (+44 vs 0 or whatever it is where you are) at the start, or perhaps business extensions. the phonebook allows for one digit discrepency and still recognises the same number. if you have the same number saved under two different names it usually won't show either name just the number. afraid i have no solution to the issue though...

edit: actually i did think of one possible solution, have you tried saving one number with international format and one with domestic? it may just fool the phonebook, on the other hand it may fool it so much the wrong caller name shows up depending who calls you!