Language is the biggest influencing factor, at least for me.
I get zero ads if I search for “trash can” but an ridiculous amount of ads if I search for “Mülltonne” (German for “trash can”), all because I’m searching on http://google.de/.
Also, it depends on what country Google have geo-located you as being in and not on what country's Google page you're searching from; non-Americans get completely different search results on Google.com from Americans.
I get zero ads if I search for “trash can” but an ridiculous amount of ads if I search for “Mülltonne” (German for “trash can”), all because I’m searching on http://google.de/.
Screenshots: http://imgur.com/a/Yco1R#0
(Hey, the German results leak the city I’m currently staying at. The first organic result is a form to sign up for the city’s garbage collection.)