I've encountered many companies that let you unsubscribe, then add you to a 'new' mailing list a few months later. You can usually identify these companies because when you click to unsubscribe they take you to a page with a dozen or more 'newsletters' that you have to uncheck to remove yourself from if you can't find the 'all' link.
Unsubscribe also frequently doesn't carry over when a company is sold to another, who then harvests every email ever mentioned anywhere to send new spam to.
I've encountered many companies that let you unsubscribe, then add you to a 'new' mailing list a few months later. You can usually identify these companies because when you click to unsubscribe they take you to a page with a dozen or more 'newsletters' that you have to uncheck to remove yourself from if you can't find the 'all' link.