Hello Vr46,
I liked that someone though of asking such basic but very interesting aphorism. I will try my best to explain it. The answer lies to basic of statistics, i.e CLT (central Limit theorem), which tell  arithmetic mean of a large number of samples which are independent of each other or random variables, each with a well-defined expected value and well-defined variance, will be approximately normally distributed, regardless of the underlying distribution. And more the sample, lesser the skewness in normal distribution curve. I will tell you in other way, that individually everyone may have different perception for current market price, but when we act(participate in market) final price is the normal distribution of every person participating in market. Hence individually we may be wrong but collective we become right. To make you this theorem clear. I am sending you link from youtube, which show the experiment based on large sample size and the price prediction. https://www.youtube.com/watch?v=iOucwX7Z1HUÂ