I have noticed this as well and could not find a way to change this behavior.
On a side note, I also noticed that the 3rd circle is always on top. Therefore, it is visually most pleasing when the smallest circle is the last circle, due to the transparencies of the colors.
By the way, since you're looking into venn charts too, do you by any chance know more about the question I posted yesterday?
Cheers,
Stefan