Hi, now it is clear to me:
If you create a closed curve with the freeform line-tool and fill it, the ai export is kind of identical, no matter if you export the curve or the surface edges, you see closed curves in Expression.
If you, instead, create a "closed" curve by connecting several curves and fill it, the export of the curve is a closed curve in one piece, while the export of edges is one "object" containing the pieces of curves. I would not call that a problem, you just have to know about.