Question: How did X get tenure, five years ago, when I did much more than X did and my tenure did not go through? Other versions of this question: x, y, z got tenure having done this much, with average letters; yet A did not get tenure, and his dossier is manifestly much stronger. My colleagues are demoralized by this fact, in part by x, y, and z's weak performance, but also by A's denial and he is so much better.
a. There was a change in Provost in between.
b. Mistakes are made all the time. Hopefully not too many, but mistakes are made. Why are they made? Sometimes judgement calls turn out to be wrong, sometimes a dean makes a plea for someone who then does not pan out, sometimes the dossier is very well put together or the reverse.
c. The University has become stronger in terms of its faculty. Everyone seems to agree. It could be that those tenured years ago rose to the occasion and en masse moved the institution forward. Or, it could be that the newest appointments and tenures are qualitatively stronger. Or, both? And many of the past tenures have not moved forward. No one suggests that in general the past tenurings given the records at that point would now go through, although it might be argued that those people could have performed much more strongly at year six if the expectations were higher (no one seems to make this argument, either).
Most people agree that our demands at tenure time are more demanding than they were a decade ago. And there may be greater reluctance to give a pass to marginal cases. They are probably more demanding than five years ago. In part, we compare our candidates to peer or better places,and as the University has grown stronger, those peers are in fact better. More to the point, candidates are compared to peers elsewhere, not to colleagues in their own department.
d. Sometimes a more comprehensive picture of someone may be revealing. Perhaps their whole department depends on their teaching. Perhaps they are good at getting grants. Perhaps there was a spousal hire issue, and the candidate was good enough but not stellar, but the spouse was the star they really wanted to hire. Perhaps, there were retention issues, including preemptive promotion to keep someone off the market. Perhaps, the actual letters, which you may not have seen, suggest the candidate does not deserve credit, or is much better than the record indicates, or...... Or, someone who publishes lots has antagonized all of his colleagues, and so their votes may well not be strongly in his favor. ... Or, perhaps the research or style does not match well with the department, and the case is marginal, and so there is little enthusiasm for giving them a break. They then might go elsewhere and be considered prizes--since the match is much more appropriate.

In general, what one should do is to focus on becoming stronger, rather than comparing oneself with weaker colleagues or tenurings. You will then be in a better position to get offers from elsewhere, and your dean may have to pay attention. If you present a marginal case, and perhaps half are in that form, you are setting yourself up for problems. Rarely is a strong case or a weak case mistakenly assessed. The best defense is an offense. And if all else fails, Living well is the best revenge. Go elsewhere, thrive, and don't look back. Find a better match, where they appreciate your talents and virtues. (Similar advice applies in matters of love and relationships,)

By the way, to prevent a marginal dossier--do the work you are expected to have done by the time you come up for tenure (including having the articles or book in print!), make sure you do a decent job of teaching, discourage your committee from geting letters from your advisor or collaborators or other previous employers (except to testify to your contribution to joint work, or your teaching or colleagiality at a previous institution). You want you dossier not to have any flags that say something is fishy here. If you have some untoward events in your life or career that affected your record, deal with it in you personal statement in a sentence. And the first paragraph of your personal statement should summarize your contribution to scholarship and suggest your future research trajectory as you now see it.