I'm just taking a stab in the dark but maybe the other 3 wheels have been repainted and that one still has the original coat which has been corroded by brake dust. That would explain the lack of rock chips on the other 3 wheels.
I didn't take enough care of my OEM wheels and they looked horrible with all this baked on brake dust. No matter how hard I tried I could not remove it. Someone crashed into me and I had one wheel repainted and I found that the repainted wheel was much easier to clean because the paint on it was fresh.
I've now since painted them all flat black and last weekend I took them off and nano sealed them with Gtechniq C5 Wheel Armor. Hopefully that'll keep them nice and shiny and stop that pesky brake dust from eating into the coat.