it’s the novel art, a different artist is drawing the manhwa because the novel artist cant
It'a basically a book cover. Single illustrations are usually of higher quality to a comic if only because comics are in a certain style. So that they can be made as quickly as possible or as quickly as they can to have a scheduled update ready.
The artist has to be able to post quickly but make the panels good. Most likely they took more time on the cover to make the series look good since they don’t have the time to make the art that way every single pannel since they need to update quickly.
the picture of finea and the prince on the last panel is better then the actual art of the manhwa... whenever i see that last panel i couldn't help but compare why is the art's better..(⊙…⊙ )