I'm not sure that 'inaccurate' is the best term for describing the lines on the mats. It - at least in my mind - goes to the principle of using the same ruler (or at least same brand) throughout your project. Not all rulers are created equal although you would think an inch is an inch is an inch. Just check the measurements on your different brand rulers and compare them with the lines on your mat. You'll see that you have slight variations between brands. If you're using an omni grid mat and omni grid ruler they are likely the same or very close. But different brands can be different. Plus the lines on the mats are generally thicker than those on your ruler which could also account for slight differences.