No machine is perfect, and sometimes there may be a very minor offset between the front and the back. We always do everything in our power to avoid this, but we allow ourselves a margin of one to three millimetres maximum offset. Link to this answer: https://old.azaogames.test/en/faq/the-front-an-the-back-of-my-product-look-out-of-line-why-is-this/