This is not a mistake. REPEAT: This is not a mistake. The extra lifespan you see from the Art of Longevity is not cumulative. The +300 means that he has 300 extra years on top of his original lifespan. His original lifespan was 56 so it is correct that his current lifespan is 356. Again, the effect that is shown is not added to the last effect shown.
[Lifespan: 18/356 years.]
Eastern · Crustacean
His eyes, which were previously closed due to the pain, slowly opened. Jason's previous lifeless eyes were replaced by two brightly shining and vigorous golden eyes flickering in the darkness.
Urban · HideousGrain
Hmmmmm
A male student beside Wang Teng was waving his fist around feverishly. Even the pimple on his face turned slightly red because of his excitement.
Eastern · Don't Enter The Jianghu
And also their children's children's children's children's children's children's children's children's children's children's children's children.
Getting this skill book was not easy, but it was the only way to awaken if you did not awaken naturally. The rich had no problem awakening themselves, their children, and their children's children
Infinite Mana in the Apocalypse
Games · Adui