Something to add to the answers already given... this isn't really what you asked, but I'll answer it anyway just in case.
One resource can give you +2 or +2 in certain cases, if you build the right building. When I first started playing Civ4 I was going crazy trying to figure out why 3 Furs gave each of my cities +1 but 1 Silver gave each of my cities +2. Well, I build Forges like crazy, which give an extra happiness from Gold, Silver or Gems. I played several games confused about that until I accidentaly stumbled across the answer in the Civilopedia. It also explained why 1 Fish would give some cities +2 and other cities only +1- my coastal cities had Harbors (that double the for seafood resources) and my inland cities didn't.
This is why the BUG (or BAT) mod is soooo good. Hovering over buildings will provide all this info, like harbors giving +1 for fish, clam and crab, and additionally telling you what you actually get ("Actual +2" cos you've only got fish and clams, for example). This is good for custom houses, because sometimes you 'actually' get no bonus, as all your trades routes in that city are domestic.