NSFW and NSFL can mean different things to different people and cultures.
Best example is that in some countries, showing a boob isn’t straight away NSFW as much as it is in America.
Opposite example: In America, showing blood is almost G rated at this point (at least it feels that way to a non-American watching American media). In China you effectively can’t show blood in any media.
As has been mentioned in the comments already one solution would be more specific categories, probably customizable by site admins to suit their particular site culture. Either that or a site can stick to just a single NSFW tag to donate a more “hey, this might not be the best thing to look at”.
The distinction is made to differentiate between two very different and clearly separated categories, one being just inapproriate to show around (a boob on your phone in a public is inappropriate pretty much everywhere, not just in the puritan US) and straight disturbing content that you might never want to see, even in private.
Different customizeable categories would be needlessly complicated to implement in every client
