There are many reasons why AdSense systems might attribute a few impressions to sites that you do not control and that do not directly have your ad tag on them.
- Sites that serve the contents of your site from their own domain
For example, Google cached search results and Google Translate may retrieve your content, then serve it from a Google domain, without iframes (in this case the content is shown through webcache.googleusercontent.com and translate.googleusercontent.com).
- Improper iframe redirects
For example, some sites create an iframe through which they reference another site.
- Web pages forwarded in email clients
- Duplicate content hosted by multiple publishers
Copy and pasting, or otherwise duplicating, stories from one publisher to another can cause this issue.