OK, Check the valve lash clearance on #6. If the lash is tight, adjust and check the compression on #6 again.
Squirt some oil into all of the cyls. and check it again. Compression in #1 - #5 will probably raise a little. #6 will probably raise a lot. This would indicate a problem with the piston rings sealing properly. If #6 raises about the same as the others, the top side is the problem. A bore scope in #6 might tell you more.
My gut feeling is the same as grannyknot's. For some reason the rings on #6 haven't seated. You might want to find out what kind of rings were used on the build. I don't have much knowledge about rings but IMO moly rings are the most forgiving during break in. Chrome rings are considered a step up from moly but they have had problems seating and sealing. I've always used moly rings for the street. Another plus with moly is they will reseat, even on on old engine that has been sitting for a long time. "Surely" there are members that can add their knowledge about rings. I think the discussion is about to arise.