When walking in one direction with only one coordinate changing, it will increase and decrease while walking in the same direction or not change at all, meaning that walking 10 blocks will decrease to 5 then increase to 20 or not change, but in reality. you have only walked 4 blocks.
included screenshots from the nether shows the x-coordinate rapidly increasing while moving towards a still pigman, then after passing him, the x-coordinate stops even after moving away from the pigman.
You're moving in z direction, not x direction.