Have not reviewed all of the responses to your post, but 33 mpg is pretty normal for around town riding. The engine has a very heavy flywheel, used for vibration damping. That flywheel has to be accelerated along with the other moving parts of the engine, and that takes fuel.
My Ducati weighs about 45 lbs empty less than the Bonnie, and both are small bikes. The difference in weight is almost entirely in the engines alone.
Once I'm out of the open road, my Bonnie really shines in the mpg department; I'll generally see better than 50 mpg. Yours should be about the same, unless you have a really heavy throttle hand.
I see the same numbers with the Ducati, with its precise Marelli ECU, fuel injection, and dual ignition. The Duc also has desmodromic valve actuation, which makes for better fuel efficiency, so the Bonneville's gas milage is still impressive by any standard.