Window frame construction is weird

[Donovan]

I am using HD Suite 2014 for a renovation project so I'm trying to build an accurate model of my house in its current condition. Since I couldn't find anything saying whether window dimensions refer to rough openings or finished openings (it's the rough opening) I decided to do some detective work. In the process I discovered that HD seems to have a couple of funny ideas about how windows frames and trim should be installed.

I created a 6"x6" window with 2" casing and set all of the sash dimensions to 0". Here's the result shown in elevation view:

Note the thin yellow line on the top and sides, that's a 1/8" reveal. Now, note the thicker yellow line at the bottom. That is a 3/4" bottom window frame placed on top of the sill. Isn't the sill the bottom member of the window frame? Why would there be an additional frame piece on top to the sill?

Here's a somewhat larger version of that window in 3D view that shows it a little more clearly:

This is a 24"x24" window with the sash dimensions all set to 1". The yellow box set more deeply in the window is the sash. The yellow nearer the casing is the frame. Note the large piece across the bottom.

There is a second weird thing when viewed from the exterior:

This is back to the 6"x6" window with 2" casing and 0" sash, again in elevation view. The dark blue is the casing. The pink is the full 3/4" of window frame visible inside the casing on all 4 sides. Shouldn't the casing cover the frame?

So am I missing something here? Do I have a setting wrong that is making it do this or is it a bug?