Why does grout for new wall tile darken upon getting wet, after my installer claimed he sealed them?

Is this normal depending on the type of sealer...or were they not sealed or is moisture from the tiling and/or grouting still trying to get out? He waited 2 days after grouting to seal. I wasn't around so I don't know for sure if he really sealed them, but water does bead up on the tile itself in spots.