Tons of reasons the left might be different from the right. These were made in pieces and put together after the fact. The left may have been in a container against this wall out if the sun, inside a cooled storage vs not, the plastic composition many be different, the die to color the plastic may be different, etc... there are thousands of things that could go into this, we don't know. The treatment these recieved at the factory before they were ever molded when they were still raw plastic could have been a trigger to begin the breakdown we see now. There is just no way to tell. Without a untouched scientific sample from raw plastic all the way through mold to paint to store to play wear we just can't say at what stage chemical decomposition began, all we can say with certainty is it is decomposition as the color changed from what it should be to what it is.
The fact that one arm did not break down as much as the others (if any) is due to the way these were made. Just an example but say Monday they molded 10 arms and ran out of plastic, Tuesday they put a different batch if plastic in and molded 10 more arms. All 20 arms go to the same barrel waiting assembly, in assembly worker x reaches into the barrel and randomly pulls an arm out now worker x may have grabbed a Tuesday arm while worker y grabbed a Monday arm so the plastics were different but at the time they appeared the same. It isn't until now, we see one plastic degraded more than another.
I have caviar dreams and McDonald's pockets