When tile is fired, it is not always 100% flat and you notice this more in a larger tile. The high point is in the middle and the low point is on the ends. Therefore, if you offset at 50% the highest point will meet the lowest and will will notice this more. There are industry standards that need to be met so that there isn't too much "bowing" in the tile.
Running bond or brick pattern tile should always be offset 30% so this difference in thickness is not noticeable.