I've been commuting year 'round for 12 years, and I'm not at all surprised. A bicycle, particularly the kind of bicycle you can buy in the United States, is not inexpensive transportation -- particularly if you use it in all weather. I contend that the marginal (not average) per-mile cost of operating a bicycle is generally more than operating an automobile.
On the other hand, think about walking. If you walk for transportation, you'll be lucky to get 1,000 miles from a pair of $150 walking shoes -- a marginal cost of 15c/mile. Add to that the cost of clothes and accessories, and you'll find that from a marginal perspective walking is no cheaper than driving.
What makes driving expensive are the fixed costs -- depreciation, financing and insurance. From a social policy standpoint, this is problematic, as at an individual level once you've decided to spring for a car there is very little incentive to limit your driving, but at a collective level all of that driving imposes a burden on society and the environment.