The 7-point rule really is not directly about credibility or dishonesty or manipulation or pride, or anything like that. It is, in large part, about judging the beer properly and giving the entrant a fair score.
As has been mentioned, a great deal of subjectivity goes into judging a beer, but as also has been mentioned, a great deal of effort has gone into limiting that subjectivity. If two judges cannot score a beer within 7 points, then too much subjectivity must be present. That is when the judges need to discuss what is going on and the reasons for each score. Sometimes, it is a matter of "I like this one more," sometimes it is a matter of "I can't pick that up," sometimes it is a matter of interpreting the guidelines, and sometimes it is something completely unexpected. Usually, a brief discussion resolves the issue.
Experience plays a large part, as well. Each judge has to know his own strengths and weaknesses. I know that I am hypersensitive to DMS, so I have to be careful about how hard I ding someone when I pick up DMS inappropriately. I also know that I am hyposensitive to the flavor and aroma of diacetyl. While I usually can pick up the diacetyl mouthfeel, I need to listen up when my fellow judge mentions diacetyl, while I do not detect it. Experience teaches you how to deal with your own palate. Those judge discussions also, often, are some of the most valuable in one's education as a beer judge.
In the end, the judges make up a panel, not just a survey of individuals. If the panel cannot arrive to a consensus of 7 points (a span of 14% of the entire scoring range, for crying out loud), then something is wrong with the panel, and the Judge Coordinator needs to get involved. I have seen that happen at least a few times, most notably when two judges could not agree on whether the beer had an acetobacter infection (one judge believed so, and the other thought it was "yeast character"). They took it to the Judge Coordinator, a couple other experienced judges were brought over, and the matter was resolved. That is just a matter of getting a fair score and a fair evaluation to the entrant. I hate to think what the entrant would have thought had he received two scoresheets with very different scores and notes. I also hate to think what would have happened had the incorrect judge continued to judge beers incorrectly in the same manner.