Because ZUN can't babysit his artists, just like any other professional who works with artists.
Hell, we have proof that it happens. The artist for SSiB got the Lunar weapons completely wrong, and by the time ZUN found out it was too late to change it. Reisen's Bun Gun in LoLK, being drawn by ZUN, is probably a better representation of what they should have had.
Serious answer, her eyes change color because not even ZUN cares about her.