I assume you refer to the Structural Concrete skin.
What I think may be causing a confusion here is that Fills generally have a bitmap and a vectorial representation on screen (in output there is only vectorial). If you go to the Fill Types Dialog you will see what I mean. The models shows the vectorial hatching by default (you switch from one to the other using the View>On-Screen View Options>Vectorial Hatching toggle).
The Composite Structures Dialog shows the bitmap representation in the preview on the right, while in section you see the vectorial representation. Probably this is why it seems that they are not the same.
The bitmap representation is an 8x8 bitmap which does not let you define detailed patterns so the bitmap is always rather simplistic, which can add to the idea that what you see in the Dialog and what you see in section are not the same.
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