CAPTCHA - when security verification doesn't make sense

CAPTCHA is that little word or photo verification prompt you get on websites to confirm you're a human and not a bot.

The one I get a lot is the "choose the three cars" and than click ok.

But I swear they throw wrenches into the mix.

The other day I got four cars and had to select three.

How is that supposed to work?

I wasted so much time wondeing what to do.