From my experience at blekko, 99.9% of the "people" who go deep into the results pages for a single query are actually bots. You're a very unusual user, and there are a lot of bots.
There's a difference between going deep into the results, and progressively refining a query. The former is pretty indicative of bot behavior -- humans rarely go past even the first page of results. I do the latter all the time, and this frequently gets me Google's captcha, especially if I'm doing something like using site: and inurl: operators.
I've managed to trigger Google's bot detector too while trying to find documentation for a certain bank api (legitimate reasons, we were supoosed to integrate and their docs didn't make sense).
I run up against this all the time. My browser is fast from blocking all third party trackers and scripts. My searches are faster, so it thinks they're automated.
