Actually it's not google doing that, but the advertisers on Google. Any advertiser worth his salt runs variations of the same ad. After a while you drop the one

Actually it's not google doing that, but the advertisers on Google. Any advertiser worth his salt runs variations of the same ad. After a while you drop the one with lowest CTR and create another one. Keep doing this and you get a feel for what gives you the best CTR for you.