Importance of Following Procedure
Those instructions aren't very clear!  I read step #7 this way:

7: Cut notches in the seat posts. Here's hints on how to do it:
   a) Don't assume both braces are the same size.
   b) Use the seat brace itself to mark the area to be cut out.
   c) After you've marked it, start the notch by making a cut with a circular saw.
   d) Now that you have a nice cut, use a router to hog away the rest.


This is the notch they are talking about:



Does this help?