Stitch Confusion

I've just started working on a sampler to practice my satin stitch so it matches the one on my door hanging. Unfortunately the stitches used don't seem to be straight satin stitch.

If you compare the two the door hanging seems to have a herring bone effect down the centre where straight satin stitch doesn't, even when done on the diagonal. I've tried putting a long stitch down the centre and weaving into it but that just adds bulk

Does anyone know how this effect is created?

Can you see that each block of colour seems to be worked in towards the centre line. However on the back of the fabric there are no stitches in the centre.

My attempt to recreate the stitch. The white area was done with a long stitch down the centre to create a ridge but it doesn't have the same effect.