The instructions are correct. The strongback needs to be an inch shorter than you would expect.

The primitive diagram below might help to explain.
Imagine each section on the ruler at the bottom is 12".
Imagine the vertical forms are 1/2' thick.
You need to shorten the strongback by the thickness of a form at each end. ie 1" total off the length.
The gap between the middle forms is only 11", but the only distance that matters is the distance between the form edges closest to the ends of the strongback.
You will notice that they are each 12"

All of this assumes you fit the forms attached to the stems on the end of the strongback. That is the recommended way and the way I did my Night Heron.