Tolerances are the sizing, i.e. if they state a tile is 300 x 300 x 10mm the tolerance might be + or - 1mm or less. When i called tile mountain they told me there's is + or - 3mm on the 300 and + or - 6mm on the 600 x 300 granite. The thickness was shocking as well varying from 5mm to 14mm. It was like sorting and laying un-calibrated slate. Hope this makes sense. It can cost you a lot more to stick down bad quality tiles due to man hours etc so it's not always the purchase price that is the only factor.