[CobraMarty]

Most interesting.
I took my single stage and 3 stage intakes off the shelf and looked closely at them.
I looked down them with my borescope, mostly thru the MAP sensor port.

3 Stage Intake Manifold-
Stage 1- both disa valves closed, divides the manifold into 2 separate 3 cylinder manifolds. Front bank and rear bank. Longest intake tract, more torque/power.
Stage 2- the smaller disa valve opens and there is a small about 1"x1" crossover pipe which opens and allows the 2 sides/banks to communicate. This allows one bank of cylinders to draw a small amount of air from the other bank and allows the 'frequency resonance wave' to pass to the other bank of cylinders
Stage 3- the larger disa valve opens in the large plenum between the 2 banks and lets any cylinder draw from both 'horns' and shortens the total runner length for more power.

1 Stage Intake Manifold-
-the small 1"x1" crossover tube between the 2 banks is open all the time(just like stage 2 of 3IM)
-the large plenum connecting all runners, in the middle there is a metal plate separating the 2 banks but with about 30- 1/4" hole drilled in it. This allows a very small amount of air to pass between the banks. It is not blocked off completely(like stage 2 of 3IM) or wide open(like stage 3 of 3IM) either.

So it looks like, the 1 stage IM is about a 'stage 2.5' of a 3 stage IM. The '.5' is because of the large plenum partial opening by the 30x 1/4" holes in the metal divider.
The 1IM is like stage 2 of the 3IM(small disa open) and with the larger disa valve partially opened(30-1/4" holes)

1IM is Not 'stage 1 of the 3IM' like I had thought with both disa valves closed(blocked 1"x1" connecting tube and blocked large plenum). My bad.

So that is why the 3IM makes a bit more power down low rpm and more power up high rpm.
Both the 3IM and 1IM make about the same in the middle rpm.