I bought carpet last year and want more of the same carpet in an adjoining area. They told me it might not match. Why not?

Any material that has been colored artificially will have what is called a "dye lot". That means that all carpet that was colored at the same time, by the same colorant, will share a dye lot. When they mix up a new batch of dye, it is a different dye lot. Carpet purchased at different times usually has a different dye lot and it may not be an exact match. 

The same is true for other products as well. Tile can have a dye lot. Even natural wood and stone products manufactured at different times can come from a different source causing slight variations in color.