OMC / 200hp Johnson questions

This is about the V 21 that Reel looked at for me. It has a 1996 200hp Johnson Ocean Runner and the power head is suppose to have been replaced with a new one in 2006. When they did the compression check on it the numbers were:
#1 - 95
#2 - 80
#3 - 92
#4 - 80
#5 - 85
#6 - 76

The compression on #6 seems a little too low and when they pulled the plug on it Reel said there was a lot of carbon build up. Bare in mind this is suppose to be a new (well, ok a 1 year old) powerhead. Is there some kind of break-in time or service that is needed? What are the possibilities for the low compression? What might cause the carbon buildup? What should be looked at / checked out based on these numbers?

Reel did say when they took it out for a ride the engine ran good, but they think it might not be propped correctly since they couldn't get it above 4600 rpms.

Any help would be greatly appreciated. I'm seriously considering buying this boat, and want to make sure all things are addressed before I make an offer.