Or does the Star Guardian Janna splash art look a ton like Lunar Goddess Diana's? https://4.bp.blogspot.com/-Ap8zmC-xqUk/V-BRsyQV0vI/AAAAAAAAUnw/VZ6PPfQp5Rwsbv_Y9hV5l28eCmhSbYSogCEw/s1600/Janna_1920x1080.jpg http://1.bp.blogspot.com/-f359_g9FBns/Us4a7WLvNzI/AAAAAAAAK9o/DOhDetnDTJk/s1600/png.png
The Star Guardian Janna splash is too beautiful to question. 
What if Diana is a former member of Star Guardian?
I think that is one of the better examples of League's differing art styles.
The only thing they have in common is that they're both in the sky, and they're both purple. It isn't even the same shade or lighting though.
It's not just you. However, the reason is simple: purple is not a common color scheme, especially since both shades are close to each other, the resemblance due to color is quite strong. They also have similar forms, and the backgrounds both have clouds. Those similarities alone are enough, even though the poses and themes are contrasting.
