Probably despite the sealing, some water penetrates the granite, staining it darker. It makes sense that the most used ares show this the most. The way to tell if a stone needs sealing again; or, if it has been sealed, is to put water on it. If it is sealed the water will not penetrate the stone and can to be wiped off in a few seconds without leaving a damp spot where applied. The sealer should prevent liquids from entering the porous stone and provide a period of time to wipe off any solution that might leave a stain or residue. Don't use harsh chemical to clean stone as it will shorten life of sealer. A solvent base sealer works better that a water base sealer and can last 3-5 years. The solvent sealer is expensive and goes a long ways. If you were promised some sort of warranty, I would insist the installer have some one come out and seal it again.