Unfortunately, there are ways to show nsfw content despite having the setting. It got better after the "nsfw" tags got removed from the recommendation box. But you still can browse a tag that is considered nsfw and circumvents the setting. Also, you might encounter games in user profiles. The setting does not work there to my knowledge.
Many people just forgot that they had the setting on or that they uploaded a mature game themselves. This was also general advice for anyone reading here.
In some cases there is even discussion, what is considered nsfw. It is a vague description, because it is highly subjective and depends on location. It does not translate well into (international) age categories. What might be allowed for 12-16yo elsewhere might be considered 18+ in other places and vice versa. Especially for horror games.
Not intended for underage players would fit it better, imho. Hah. Drinking age in US is 21, so a game featuring beer should be 21+ ;-)