They seem to "bleach out" from exposure to light, for one thing. Many times you'll see the right-hand one bleached but the left-hand one still black, perhaps from being carried in a holster. I'd suppose being displayed with the right-hand side exposed might do it, too.
The old model grips are Black butaprene-rubber and they fade in the light, especially sunlight. Very often a gun used by a right hander and holstered on the right will have a far more faded right grip panel.
The old Colt and S&W black gutta-percha rubber grips also faded in this way.