Documentation should appear to the reader as being written by one single person. Uniformity reduces the chance of astonishment. The principles applies to all areas of documentation, including style and organization.
A program should follow the `Law of Least Astonishment'. What is this law? It is simply that the program should always respond to the user in the way that astonishes him least.
Geoffrey James. The Tao of Programming. 1987
People are part of the system. The design should match the user's experience, expectations, and mental models.
J. H. Saltzer, Frans Kaashoek. Principles of computer system design: an introduction. 2009
Same things should be handled the same. Similar things should be handled similar. This way readers have an easier job to locate the information they are looking for.
Readers expect a common structure for documents and the documentation. Diagrams and language should be similar throughout the documentation.
Consistency increases the predictability and reduce distraction. Systems following this principle are easier to learn.
References
More information on this principle.