Hey Nomad,
I'll try and explain...
In the picture below you see the basic frame work for the aft cabin; two transverse floorbeams and two longerons, more or less in the longitudinal axis direction.
The two floorbeams should be sitting with their aft side to the fwd side of the adjacent frame, see the green arrows in the picture, where the aft beam sits properly against the frame. The forward beam however doesn't sit against the adjacent aft frame, but instead sits with its fwd side against the aft side of the frame, see red arrows. The blue arrows then show the to-be situation, where the aft side af the beam sits against the fwd face of the adjacent frame, just like his little brother on the aft side of the aft cabin.
I think the root cause is mainly twofold: the frame itself measures 82mm, that does not match the intended pitch of 80mm, as indicated by the jig. Secondly, I know now quite sure, that I have some poorly positioned frames, paying insufficient attention to maintain the pitch between the frames along the entire circumference of the frame.
I hope you now have a somewhat better understanding of the issue.
View attachment 310091