Clustering is possible, yes. I've literally managed to roll natural ones in a row in real life, and probably all tbt gamers have stories of how they rolled ridiculously low repeatedly to hilarious results. We've all played yahtzy (actually I haven't played yahtzy since I was like ten but if I remember the rules correctly it's a game about getting clusters right?) even if that is a d6 game so it's more likely to get similar results. Regardless -- with an infinite number of rolls it is unavoidable.
But it should not be the norm. If the clustering causes the results to form visible sine waves then the RNG is dissatisfactory.
