A man in a red shirt and a man in a brown shirt smiling at the camera and holding up a sign.
Two men are throwing rocks at policemen.
The men can either be throwing rocks at policemen or smiling at the camera and holding up a sign.  Not both
Two men are not a man.
The biggest difference here is the supposed tone of the personalities of the subjects. In the one there a man holding a sign is a supposed peaceful demonstration. In the other the men are throwing rocks and appear to be angry. You cannot be peaceful and angry at the same time. There is also a difference in number of people. One man is not the same as two men in number.