Laravel Mutator called 9 times each time you make model creation or update

I am using Laravel Mutator in the Category Model to upload an image when creating or updating the Category

class Category extends Model
{
    protected $fillable = [ 'banner','name' ];

    public function banner(): Attribute
    {
        return Attribute::make (
            get: fn ($value) => asset('images/category/Banner/'.$value),
            set: function ($value) {
                if(is_file($value))
                {
                    Log::info('count');
                    $fileName = 'Banner_'.rand(11111,99999).'.'.$value->getClientOriginalExtension();
                    $value->storeAs('images/category/Banner', $fileName );
                    return $fileName;
                }
            }
        );
    }
}

when creating a new Category -

Category::create([
    'banner' => $request->banner,
    'name' => 'test category'
]);

the Log count called 9 times ,which cause the image to be uploaded 9 times ,

but if I used the Mutator the old way like this

    public function setBannerAttribute($value)
    {
         if(is_file($value))
         {
             \Log::info('count');
             $fileName = 'Banner_'.rand(11111,99999).'.'.$value->getClientOriginalExtension(); // renameing image
             $value->storeAs('images/category/Banner', $fileName );
             $this->attributes['banner'] = $fileName;
         }
    }

the Log count called only one time , and the image upload only once

so why

public function banner()

is being called 9 times