Despite the various cultural beliefs and diverse social environments people live in, there are some universal things to all social systems. An example would be the seven universal facial expressions: happiness, sadness, fear, disgust, anger, contempt and surprise.
If every argument is true and faultless, disagreements defy the purpose of having arguments. Disagreement means that one side finds the other incorrect or is offering a less likely explanation of the phenomenon discussed. People disagree because some knowledge or arguments are more accurate or correct than others. 
Such disagreements are valid, which shows that evidence and knowledge are not impossible.