You need to click on every image that contains even a small part of whatever they say to look for. I quit using Captcha on our web site a couple of years ago and went to questions if anyone wants to join.
I've wondered about that too. I look for "buses" in each image presented, but rarely find more than one, and usually only a partial image of just one single bus. Same with carS, bicycleS, bridgeS, crosswalkS, motorcycleS, fire hydrantS, etc., etc.