You have probably created a configuration and noticed that some of your posts have not been affected. This is because the settings made in the best excerpts will affect only the posts you create AFTER that configuration.

Posts are affected by the configuration at the time they are “inserted” into WordPress.
This means that it affects the entry at the time it is saved in the WordPress database, so changing the settings later does NOT change the previously saved posts.

There is a mode that does affect all extracts simultaneously “live” because the filters are implemented at display time, this is functional for not very large sites as it needs to implement more queries to the database and while this is always relative to the cost/benefit ratio of what you get, it is not entirely good for site performance. Since it adds the filters and extract modifications at the time the page is loading.