Tiles are fired in a kiln, and small size differences between tiles (and between batches) are a normal part of manufacturing. They're allowed for within British and European tile standards. Your tiler allows for this with the grout joint. Rectified tiles are ground to a more precise size after firing, so they vary less.
If you have any concerns about the size of your tiles, please send us photos and measurements before you lay them and we'll take a look. We're unable to accept claims once tiles have been fixed.