Pilou,

I would think it's because offset would create a self intersecting curve for the 5th offset, to small a radius I think. Ideally, it would do as CorelDraw does on the free form shape (see video link above) but this would require a serious (time consuming) overall to the offset command, basically an all new command would be needed for that. If you take a look at the free form shape in the video, the offset command (as it is now) would fail a long way before it reach the center of the shape as CDraw does. It's a totally different creature then offset that would be required.

Thanks,
Felix