In this kind of "tough cards" situation it seems that there are two overrepresented categories (here, doubles and ovals). So the cards that counter those attrib

In this kind of "tough cards" situation it seems that there are two overrepresented categories (here, doubles and ovals). So the cards that counter those attributes (e.g. half-shaded singles) would be able to make many sets. But since a card can only make one set and take off two other cards, the ability of those cards to rebalance the tableau is limited. So when you get in one of these situations where the next three cards are feast or famine, it tends to persist. Having a persistent cluster like that gives you a lot more chance to run into the no-set case.