There is a reason you were talking about what you were talking about, usually location based.
I once stepped into the Shelving furniture aisle in a department store to take a 40 minute phone call from an attorney about his website.
The next several days I got a LOT of ads for shelves. Nothing special about websites and legal stuff. Google has maps of stores interiors, and combines them with precise location services.
My brother in law, when we shared an IP address received a PDF from a home inspector into his G-mail inbox that mentioned the crawlspace needed to be relined.
Before he even read it, I started getting ads on Slashdot for basement lining services.
There is plenty of data you give them that they think they know what you are 'talking' about without having to actually hear a single word.
All it takes of for someone you know to google something, and for your phones to be next to each other, and they think they have a data point.
I assume that's why I get ads in chinese after I moved in next to some chinese speaking people.