Some people argue to educate and persuade.
Some people argue to win, and utilize misdirection. That misdirection is effective with subtle changes of position, facts, and emphasis.
The two groups are separated by intent. The former people are beneficial for everyone because they test theories for the sake of knowledge. They are testing their own beliefs and, in doing, are helping you test your own.
The latter people are typically motivated by selfish motives (i.e., the win).