Same people, different moment
It's easy to get hung up on demographics when you buy advertising. This is probably a mistake.
Consider this insight from Pamela Parker (Lessons from the Cutting Edge: RSS Advertising.) "It turns out
the ads on the site are geared toward an audience of people who are
discovering the content -- product information -- via a search engine.
Because these folks are at a certain, and very attractive, stage in the
buying cycle, advertisers are willing to pay higher prices. Its feed
subscribers, on the other hand, may or may not be in the same stage of
the buying cycle -- they just happen to be interested in that category
Short version: a person's receptiveness to an ad changes based on where they are and what they're doing. Google AdWords are brilliant for just this reason. People are trained to Google in order to go somewhere else. Most sites train people to come and to stay. Same thing is true with magazines.
Thanks to Tom Cohen for the link.