[icecream_so_gud]

The original reCAPTCHA, with two words: It knew the answer to one of the words and didn't know the other. At one point some users were inserting purposefully wrong answers or slurs into the unknown word to mess with the data collection.

I expect the idea is the same with images, if there are 4 buses I expect 2 it knows are correct and used as a sanity test, 1 it's pretty sure it's correct (from majority of users selecting it), and 1 has some heuristics that might be a bus but it is currently testing to see whether users believe is or not. Similarly with the 5 not a bus images.