Saltillo tile is unlike any other tile. It isn't super hard and tough like your typical man made tile such as porcelain or ceramic. It is really quite soft. After all, it is sunbaked clay and hasn't been fired in a kiln. You may even find an animal footprint in it much like you find an animal footprint in fresh concrete. Sometimes it has been left in its original, unfinished state to age and change through natural wear and tear. The color it took on as it absorbed the dirt and mop water it has been subject to for years is now part of the tile. What? Dirt becomes part of the tile? That's exactly what happens! The dirty mop water sinks right into the clay and become part of the clay and cannot be separated from it. That potted plant that sat there for years has now left a permanent ring. You can remove some of the top clay and some of the discoloration, but the stain is NOT sitting quietly on top of the tile waiting to be rinsed away. Some of it is, but not all. A lot of this added 'color' is part of the tile forever.
Sometimes, a clear or glossy sealant is applied over the tile to protect it so it is easier to clean and looks prettier. This is great. It looks nicer and does protect the saltillo against further intrusion of foreign materials. It works well if it is a quality sealer made for saltillo. Hopefully, a proper cleaning job was done before application of the sealer. If not, the old dirt is sealed inside.