It would appear to be the browser.
If I close that, then change the IP by disabling the VPN, then re-open, the ads still show.
Not currently signed in to Google, but then I don't have any Google windows open either.
I can block the googlesyndication domain but that breaks websites which still use a table based layout which then wait for whatever is meant to fill a space to time out before rendering the rest of the page.
I also have third party cookies blocked in Chrome and the ads there seem suspiciously tailored, too.
Something suggests to me that the "Block Third Party Cookies" option doesn't actually work in either browser.