This is known as the "DAS" (driver's arm syndrome). In most countries it's the left arm (as the driver sits on the left seat in the car), and it's because it more exposed to the sun than the right arm (or viceversa, in your case).
Quoting from [adseaco]:
Hello Friends,
Do you have answers to this?
My Right Arm is darker than my Left Arm!
Any possible reasons you could guess for this shade difference?
Experiment it yourself, by putting your arms together and see if you find any colour / shade difference.
It is possible, that people in many other countries too, may have this effect, but it could be vice versa.
Quoting from [future]:
Fortunately, my right arm is no darker than left one.