Hi Pilou - your top sweep surface is different because the curves you have there are made up of 2 segments joined together. Mine makes a single surface because each curve is only made up of one segment instead of 2 joined pieces.

Try deleting the middle point where your segments touch each other to fuse them into one segment which will then make one surface for the final result.

To see the difference more clearly, run Edit/Separate on your curves, and run Edit/Separate on the curves in my example - notice how yours breaks down into 2 pieces, while running Separate on mine does not make any separate pieces?

- Michael