Ok, I'll try to explain this again.
There is a formula for the way light rays dissipate. The further you move a light from a surface the wider of an area the rays cover. So, as you double the distance of the light to the surface, the rays cover an area that is the square of that distance. So, suppose you have a light 2' from a source - its rays cover 4' square. Now if you move it to 4' away, it will cover 16' square. Now, the intensity of this light is decreased by an equal proportion. Since 4/16 = 1/4 or 25%, each square foot of the 16' square will receive 25% of what the 4' square would.
Now, this assumes a light source with no reflector. A reflector creates a whole other dynamic because it stops the light rays from dispersing normally. You can see this if you have ever played with an adjustable flashlight. With a flashlight you can focus the beam to make a small area of high intensity or widen it to make a larger area of weak intensity. The loss in intensity comes from the natural spreading pattern of light, not from the net distance traveled. Put another way, the inverse square law assumes natural spreading of light. When you refocus the light you reverse this effect.
Now, the vertical setup has a light with no reflector so the light naturally spreads according to the following:Lumens/M^2 = LUX. Or lamp Lumens / meters square = Lumens per meter square. So if you have a 600W HPS @ 90,000 Lumens and a circumference of 5' and a height of 2' you get
(5X3.14)2 = 31.4' square.
90,000/31.4 = 2,866 Lumens per square foot.
The required light for optimal growth is 2,500 - 5,000 Lumens per square foot. So, a vert that is 5' across and 2' tall just barely makes it into good growth range.
Now, let's apply this to a standard flat grow with a reflector.
Front of bulb = 45,000 Lumens
Back of bulb = 42.750 Lumens @ 95%
Area = 5.5' X 5.5' = 30.25' square
87750 Lumens / 30.25' square = 2,901 Lumens per square foot
So, as you see above, there only difference between the two growing setups is the 5% difference from the back side of the bulb which is 2.5% of the total bulb intensity.
Now, where you might pick up some efficiency is over grows that use 1000W bulbs with poor reflectors. In such setups there plants directly under the bulb receive a bit more light than those on the edges where as those in a vert are all the same distance from the light. This is overcome by using light movers, more smaller lights and good reflectors.