You use a hand sewn saddle stitch, how is that different from a machine sewn lock stitch?

I use both types.  The main difference is that hand sewing is quite rare.  Below is the side by side difference from an illustration in the highly recommended book “The Art of Hand Sewing Leather”, by Al Stohlman.

Note that the lock stitch top needle thread meets the bottom bobbin thread and loops or locks and returns to the top. If the machine is properly adjusted, this loop occurs in the middle as shown above.

The hand stitch uses one long continuous thread and two saddle needles. The threads do not stress against each other but pass each other in the same hole.

When hand sewing, the thread is actually slightly below the surface of the leather, either by using a stitching groover or edge creaser tool. This minimizes thread wear. Should a break occur the other thread is still intact holding the leather together.

Using a diamond shaped hand stitching awl helps to part the leather fibers to make the stitching holes rather than the cuts made by the chisel needle of a lock stitch machine. The holes are not visible after proper hand sewing.

Machine lock stitching is ideal for long runs of stitching; like border stitching the lining on belts, and for the decorative “gunfighter” belt stitching popularized by Andy Anderson.