I think that Bill is referring to the two different piston crowns. 4 and 5 have dished pistons, while #6 if flat. At least it sure looks that way.
That's the way it was at that point in time. But, even if I left it that way, #6 would still make less compression than #4 or #5. With the 12 port head, #4 and #5 would make 13+ compression. #6 will make 8.5-9 compression. You might be assuming that the dished piston and the flat piston end up in the same place in the cylinder at each cylinder TDC. But they don't.
#6 is at TDC and almost 1/2" below deck in the previous picture. The pic below shows the dished piston at TDC. It is .070 below the deck.
At this point in time, #4,5 and 6 will make 8.5 - 9 compression. And if I put in another few hours, #1,2 and 3 will match #4, 5 and 6.
