Tiles, like other earthen-ware products that are fired in a kiln, will vary in size from one production run to the next and also vary slightly within a production run. This is a characteristic (not a flaw) of tile and other fired ceramic products. The variation is due mostly to environmental factors; fluctuations in kiln temperature, humidity level, weather & barometric pressure, operator differences, etc.
These small variations dictate grout joint width. The more variation in size, the larger the grout joint needs to be. If each tile is exactly the same size, a very small grout joint can be used.
To “rectify” this characteristic, the perimeter edges of tiles can be cut (machined) to produce tiles that are all exactly the same size & dimension. Rectified tiles can be set with the smallest grout joint width because every tile is exactly the same dimension.
This is an added step in the manufacturing process that increases cost. Generally, you can expect rectified tiles to cost more than “factory-edge” un-rectified tiles.