Webtoons was originally a Korean (?l company so that might be why their rules are different.
Either way, the 13+ age rating on most websites are there because they're public domain and the company hosting the website doesn't want to get yelled at/sued by the parents if their kid should see something they deem inappropriate (which falls under a pretty large umbrella before you approach 18+ category).
18+ is the only one that's very strictly legally bound (as far as i know) due to the risk of minors being exposed to pornography, or potentially drawn into child pornography legal problems that are a lot worse than "my 12 year old saw a man with a knife!!"
I'm not a lawyer though, and I'm digressing from the original topic quite a bit.
TL ; DR Tapastic's age rating for accounts and age rating for ad revenue are two separate cans of worms. If you have to follow one, follow the ad rev one.