You are missing the whole point. The new "easier" captchas are shown only to people that they are sure are humans already. I suspect they are looking at the number of previous captchas you have solved correctly, are you logged into gmail, etc.
You're correct. Refreshing the page a couple of times reverts the captcha to the old one with letters. So I guess they keep track of how many times they serve captchas to your ip address in an hour, and if you go over some limit they start serving you the harder captcha .