> The reason for that maybe because a circle starts when at 3 o'clock
> therefore if you split it vertically the edge goes into three, Michael ?

Yeah, the edge already has a "starting point", and if you cut it in 2 other locations that will dice it into 3 pieces.

But instead you can use Edit/Trim and the "Add trim points" option to place just a single splitting point, that will divide a closed edge into 2 pieces, between the start of the edge (which you can by the seam that runs along the surface) and the split point.

- Michael