Marcus:
I don't know the cause, but it looks like the lower windows (and upper side windows) are missing the sash frame which is shown in the upper central (transom) windows, which causes the gap. I do not get this problem (and could not find a setting that would cause it) so you might see if you can download again and reload the library. The library version is LIB12-USA-2528-Mac.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David