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 lose time on daily trivialities. Do not dwell on petty detail. For all of these things melt away and drift apart within the obscure traffic of time. Live well and live broadly. You are alive and living now. Now is the envy of all of the dead.” ~ Emily3, World of Tomorrow