I had the same confusion a while ago. And after MUCH MUCH brain torture, i concluded that you just can't keep the exhaust totally free from Carbon deposits. You just can't.
- Those people with "Totally Clean" exhausts actually "Clean" the exhaust tip with a cloth every now and then.
- There are some people with very good driving, which makes the engine burn fuel efficiently, resulting in less "unburnt" fuel getting stuck in exhaust pipe (all about Throttle Control and not Over-Accelerating the bike.)
ALL brand new bikes have their Mixtures set to VERY rich, and Mechanics mostly don't change it. Now that does result in "smoother" bike ride, but make no mistake, thats exactly what makes the engine leave unburnt fuel in exhaust hence the Large Carbon Deposits.
Do note that I said, "LARGE" deposits. Your bike is perfectly fine (seeing the spark plug) so even if you don't have large carbon deposits, there will still be enough to make your finger black.
So yeah, again ... don't worry about it I can totally understand your mind condition as i have been to same spot. But at this point, i've simply stopped giving a damn and using these bikes as "commuting".