Cold starts take a big bite out of fuel mileage as well. My daily commute of 20 miles on a warm engine results in about 36mpg. With a cold start thrown in at each end mileage drops to about 34. Overall average is 33.22, lower than the commute due to traffic in my drives between commutes.
As for accelerating fast vs slow, I believe in a diesel, slow works best. In a gas motor however, accelerating slowly means the engine sucks vacuum against the almost closed throttle plate, decreasing the efficiency. You get better mileage by mimicking diesel throttle function (no throttle plate) by accelerating briskly (opening the throttle plate) and short shifting. Hard to do in an automatic, though that may be why some manufacturers (Hundai come to mind, but not BMW) program in an abrupt throttle tip in, to max mpg on the EPA city cycle. Real world the driver compensates and mileage suffers.