Depending on the site, it could be a coincidence. Many large companies use mass market email and the fact that you received a message might be completely random.
There are several ways that a company could do this… and again, it’s not based on paranoia. You just need to understand what technology these companies have that allows them to do this.
It all boils down to related sites or sites that use related advertising.
Let’s say you visit Site A and buy something. You give them personal information, but at a minimum, you give them your email address.
Site A leaves a browser cookie on your machine, so whenever you visit, it knows that you previously bought something there. Now, Site A can tailor their site to your interests, keep you logged in, remind you of things you’ve purchased in the past, or do anything like that when you come back.
Now, you leave Site A and go to Site B to look around, but you don’t realize that Site B and Site A are the same company (think Gap and Old Navy1). Because the two sites are related, they share a database. They also share the cookie that Site A left on your machine.
Because of this, Site B recognizes you and looks up the email address in their shared database. As you leave, they send you an email related to what you were browsing.
Now, the sites don’t have to be related for this to happen. Two sites may have the same advertising partner which shares information.
Let’s say that you bought something at Site C. Their advertising partner works with Site D. The advertising partner may have access to information like your email address through Site C and may pass it on to Site D.
In that case, a company unrelated to the site that you actually did business with (say Site E) could use this advertising network to get access to your email address and send you more information.
Ultimately, getting your information is merely a matter of data analysis for companies and advertisers. It’s really not that unusual (and not that difficult) for companies to basically share information as you do business or simply visit on the internet.