Why is casing reveal spec different for mulled windows vs single windows?

[GeneDavis]

I am taking pains to do windows and framing and trim for a project with mulled units close adjacent single windows.  I have made the frames and sash in sizes that match the specified windows (Andersen series 100).  Horizontal window and trim lines are important to the look wanted.

 

Frame width of these fixed windows is 1 15/16".  Exterior casings are 5/4 x 4 (1 x 3 1/2" actual).  As you all know, clad windows have exterior casings that butt to the perimeter of the projecting frames, so if we are using the term "reveal" in the dialog spec entry to place casings properly, we enter 1 15/16" for these windows.

 

It works as expected for the single window, but not for the stacked (blocked) unit.  That unit is specified with reveal specified as zero.

 

Why this difference?

 

 

[GeneDavis]

Here are screencaps that point out this behavior quite clearly.  I placed three windows, edited all to have 3" frame thickness top, sides, and bottom.  I then moved things so two are a pair, and blocked ("mulled") them.

 

The casing reveals were then edited.  Here is what the reveal spec is for the single window.  3" reveal exposes the entire 3" frame all four sides.  This placement of exterior casing is typical for clad windows or flanged vinyl windows.

 

 

The blocked pair unit needs a different value to place the casings so they match the single window.  Here you see the reveal edited to 0, and using that value for reveal places the casing to match that of the single window.

 

 

This view shows the two windows, the single and the blocked pair, side by side, exhibiting same casing placement, but having different reveal specs.

 

 

Can anyone explain why Chief set this up this way?