They didn't change them; the challenge mode belts appear two dimensional in some model viewers because the belts have an artificial 3D "prop" (in this case, a skull) that hovers over where the buckle would be. What you're seeing in the second image is what the belt texture on the character looks like without the 3D skull buckle.
Last edited by Kaleredar; 2013-02-19 at 10:29 AM.
"Do not look down, my friend. Even in the darkest of times, there is always hope... Hope for a better day, hope for a new dawn... Or just hope for a good breakfast. You start small, then see what you can get." ~ Covetous Shen