Everything pretty much explained already, but I'll touch on a couple things.
18" wide is common measurement for shoulders and is a standard 100% IPSC. 12" width is 66% IPSC width or about chest width. If ranging an 18" wide target, bracket within the funnel left and right edges and read your target distance on the right hand side. Same for a 12" wide target but read on the left.
We included the centerline for two advantages; 1. help ensure the reticle bisects the target top to bottom to get the true width instead of being slightly canted and getting a range error and 2. So that you can range half size targets (6" and 9") with the same system. Six inch is common measurement given for head width and 9" is common for head length front to back. Plus, a 50% IPSC is also 9" wide.
For strictly PRS competition this obviously isn't needed as all ranges are given to competitors. But there are more comps than PRS, ones where mil ranging is still used for at least one stage, usually on the clock. If those targets are either of these four, speed and efficiency will be increased.
Lastly, our scopes are reportedly being checked out by some mil personnel. This flash mil concept with four different widths may prove more beneficial as distances are a direct read out.
The concept has been around a long time as pointed out already. We just tried to enhance it for common sizes based on width instead of height, and more versatility as well.
Hopefully that helps explain a bit for the "fatal funnel" as I started calling it. (Yes I know that term is used for room clearing procedures). ??