Some artists choose to interpret it as a removable accessory, especially when it gets in the way of "LOOK! BOOBIES!" images.
I really don't understand that mindset. How is the third eye a removable accessory when it's an organ that is part of Utsuho's body? Don't get me wrong, I like shapely breasts as much as the next guy - I just don't think they look good on every character, and I can think of several Touhou ladies who look fantastic with a large bust. When the third eye is such a significant design trait of Okuu's, I simply don't like seeing it removed in favor of a generic big rack.