In production ,if the stone is filled in areas, the filler can have a finer surface finish, so will the stone when first installed.
After several years the face of the stone will open up ( become rougher, open pored )with trafficking and maintenance .small micro pores will then fill with dirt giving a slightly grubby appearance . The smooth filler will remain the same and appear cleaner. This then shows up on the tile.
You can "test" a suitable alkaline stone cleaner on one tile and re- impregnate when fully dry. This may improve the look. Or perhaps have the stone re polished with diamonds or twister or kgs type pads to refresh the face and possibly cut the filler smear back. Then impregnate.
Maintain with a specific stone daily cleaner to minimise cutting the Impregnator or further damage to the stone face.