Tags are one of the most important aspects of the site, and play a major role in organizing the information here. But a tag can be created by anyone whose reputation is >300. So we have lots of tags (1,300, by my count), many of which are not very good in one way or another. They often have no wiki excerpt and their names and usages can be unclear or ambiguous. Cleaning them up, adding / removing tags from questions, providing wikis, etc. will always be a work in progress. You can suggest a synonym for a given tag if your reputation is >2500 and you have a total answer score of $\ge$5 that tag, then it awaits 5 upvotes to be ratified. Rest assured that you will wait forever. I created a meta CV question (Current tag synonym candidates) to make such suggestions more salient and open for discussion. That may be a more productive option.
At any rate, there is no real rule per se. The truth is that we have what has been created and hasn't yet been changed. I think at best we can aim for a reasonable consensus position (which in turn is likely to be continuously evolving). Ultimately, that position will be a weighted average of unawareness, indifference, and what seems to most usefully organize the information on the site (pardon my cynicism ;-). The latter can definitely change over time. We have lots of tags that are proper subsets of others, well beyond just normal-distribution $\subset$ distributions. I agree that
[standard-normal-distribution] would be unnecessary, but that is a judgment and a contingent one. The users (I am not one of them) who contribute to and follow the adaboost and boosting tags should try to decide what is best at this time.