This can happen easily when marketers try to target individuals by psychographic or behavioral data. I surf around a few pages in the site and then leave. The marketer then infers from this bit of information that I'm interested in outdoor topics and products, and subsequently begins pushing me offers of this type. This is usually accomplished via an advertising or marketing network that tracks surfer activities and puts the information in a database. The problem with this approach is that the inferences drawn are often incorrect.
Many people surf on a lot of different sites, spending a decent amount of time there.
They look at all kinds of things, but this doesn't mean they are necessarily interested in the general theme of the web site. For example, I visited a site this morning looking for a writer I thought I might have gone to high school with. I spent about fifteen minutes on the site until I figured out the writer and my former classmate are not one and the same. The site was about fashion, and anyone who knows me knows this would not be a good topic to target me on. Even targeting on purchases isn't a reliable indicator because people buy merchandise for others.