Looking through some other places on the site it appears to be based on the 2000 census.
If I remember correctly there was a place on the census questionairre I received to check off how far I commuted as well as my method of commute (Bicycle, Train, Car, Bus, Walking, ??) not sure if there was an option for multi-modal or to define what the different modes were.
College kids could consider school as work or maybe they do work on campus or after class at part time jobs. I always wonder about census data and I now work with it every day, filing endless reports to the government.

Edit: hm.. Well either they aren't showing it on their site, it is included in "other" or they actually didn't ask. When I just checked census.gov for portland it doesn't like bicycling in the results. Here is where it would be listed under "Economic Characteristics" See cycling is an economic choice per the government
http://tinyurl.com/277p2e