It's a better cammo pattern for that environment, plain and simple.
Issued uniforms are typically not of good quality. Look at the ACU, it had to be upgraded soon after it came out. Senators who have friends are the reason we have all of these uniforms. It's why the Navy got new PT gear, the black and tan uniform and the blue cammies. If the government provides us with the equipment, it's probably no good.
Edit: You will see operators mixing and matching uniforms to best fit the terrain. They do this with all equipment that they take to the field.
devin said it best. they choose what fits in best for were they are. they all have a cambat uniform that they were just around, but in theater they choose what fits best