This is universally true for almost all fan works. When someone becomes a fan, their Fan Art mainly starts out as something like simple Off-Model blobs. The fan improves their art over time, possibly leading to their fan art being identical to the official art.
A Small Crime: The writer said that originally, Kit had long-ish blond hair (instead of short black hair), had blue eyes, and that she wore red armor (instead of wearing a grayish-black outfit), but then realized that she looked too much like Artie. The red armor was switched for a shirt to make Kit less impenetrable. It was even said that Kit originally had red dragon-like wings.
Ask Hoops and Dumbbell: While the style remains heavily rough and sketchy, the artist's style has refined a lot in every other aspect over the blog's run.
Blooming Dreemurr: The blog started out with simple images of the characters followed by text. Then colored images of the characters followed by text. Then text boxes with the characters faces. Then colored faces in the text boxes! Vee's been able to make moving images more recently, but they're too time-consuming for regular asks.
Bugged Run: This comic was started in part specifically as an art exercise, and it shows. The comic goes from flat black and white with no backgrounds, simple panel layout, rough linework, and deformed characters, to having full color, shading, detailed backgrounds, dynamic layouts, and more normalized character models by page 16.
Evangelion 303: After several years drawing, Grummancat’s art has greatly improved. In 2013 he uploaded a pic that showed how much his style had changed.
Touhou M-1 Grand Prix — The first 2 contests had different artists doing each skit, with one for the judging. From the 3rd contest and on, the entire competition was done by the same artist.
The first season of Yu-Gi-Oh! The Abridged Series had incredibly bad quality, audio, and acting. The audio and acting improved halfway through the season by becoming much less stilted and more consistent, and by the end of season 2, the overall quality is practically on par with the original series. The shoddiness of the first few episodes has been lampshaded quite often in several episodes.
The design of the Diamond and Dazzle MLP fancomic changed quite a bit since its inception. Just compare the latest comic to the first.
Tintin and Alph-Art (Yves Rodier) mostly experiences this due to being a completed work, instead of the rough draft that original author Hergé left incomplete at the time of his death, though Rodier did clean up the artwork slightly after its initial release, along with adding colour.