Sometimes when I edit a question, I choose a tag but it doesn't taken into account. Is someone can tell me why some tags are changed automatically?
For example, I have edited a question by addind the url-format tag but it's changed to url tag.
Some tags are "synonyms" of other tags. If you use one of those tags, they will be automatically changed to their synonym. For example stats becomes statistics. You can see the list here: https://webmasters.stackexchange.com/tags/synonyms