IMHO, I will guess it could be a supply issue since the wheel sizes are new for Tesla Motors. Perhaps someone with Photoshop will post some grey wheels on Model X for comparison. If you need photos in various colors from the Design Studio for Model X, scroll down in this article:
Model X configurations begin three weeks before first deliveries - GM-VOLT : Chevy Volt Electric Car Site GM-VOLT : Chevy Volt Electric Car Site
OT: In the same grouping, enjoy viewing the now incorrect "White Leather Seats" on this spec sheet of my 9/1 saved order:
ModelX-2.jpg
That is why the current version of the Design Studio for Model X should be the guiding light for configuration decisions.