Because the marking scheme says it different
The answer in the markscheme is :-
- Readings taken at either side/diameter of cylinder
- Position of mid point found
- Mark position of centre
The marking scheme is correct.
Okay, suppose the diameter of the cylinder is 2 cm.
Mark the center of the cylinder.
Place the cylinder at 10.0 cm.
If it's diameter was 2 cm, you should be able to see exactly to the left of the cylinder, 9 cm, and to the right, 11 cm. This will prove that the centre is indeed on 10.