Leathercraft Techniques

For the life of me I have no idea what is going on! When I am saddle stitching leather the mains section I stitch is just normal and the back second has decorative angles, it really baffles me I done a quick example of it,

here is the front, is has some straight stitches, a tiny bit of decorative.

Here is the back (stuck veledon on it to stiffen the leather while stitching he example) but you can see angles wway more on this side?

Can anybody give me any hints to why this might be happening? (also happens on every leather i try and use at the minute)

Thanks in advance