Each of the rumor sites have contributors who provide information, and the ones who have provided accurate information in the past are given the best CR rating.
As noted, sony does not really try to keep upcoming products secret, and nikon might want to, but is not successful.
Canon, on the other hand severely punishes those who sign non-disclosure agreements and leak the information. They are very good at keeping a secret.
No one is perfect, and as soon as a product is almost ready for announcement, the number of people with information about the announcement grows tremendously, and leaks will happen.
There is a big difference between information based on prototype bodies and real ones, many prototypes are out there and few of them will become a released product.