How is that an error? What I took from seeing this is the artist wanted to portray all the digimon in their forms they usually chill in and then their higher stage(s).
Unless there was some sort of tagged that was removed before I saw this.
Not so much an error as it is "this is how it went down on tv"
Plotmon/Salamon wasn't around as much as Gatomon/Tailmon, and that being said, wouldn't look as good next to the other Adult/Champion levels with her being the size of a Child/Rookie level.