This happens when they already know there is a fair chance that you are human. So they give you an easy one and get their OCR for free. You are probably already logged in your Google account when it happens. Go incognito, browse with Tor, etc. and you will get the impossible ones.
Basically I have to fill in the number and then guess whether it was the first or second set of characters and fill out bogus before or after the number and hope I got it right. The numbers weren't even hard for a computer to read. The only thing it does is waste everyones' time.