The inside pipes have these butterfly valves which basically allow the exhaust to sound louder while also allowing for the extra 5 horse power.

It seems that either you drive mostly with the NPP exhaust off or your driving pattern and RPM ranges does not allow for butterfly valves to open all that often. hence the carbon build up on the outer pipes only.
Yes mine is always on. I like the rumble. In my opinion It was the best option I put on my car. I think I will just keep cleaning them in lieu of tickets ;)

I use the mothers power ball to clean them. It works great.
Get a Mothers Mini Power ball and a drill. Add a little metal polish and Bam instant clean..:coolgleam::canada:

Aredam Custom Detailing: Camaro ZL1 / Metal Polish Powerball - Exhaust Tips - YouTube
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