This is because there is a remarkable pattern in stock market returns, uncovered by Anna Cieslak of Duke University and Adair Morse and Annette Vissing-Jorgensen of the University of California at Berkeley. They show that US shares do far better in even-numbered weeks after an FOMC meeting than in odd-numbered weeks. In fact, they say, since 1994 all of equities' outperformance of cash has come in those even weeks.
For example, in week zero (defined as running from the day before the FOMC meeting to three days after it) the MSCI US index has outperformed one-month Treasury bills by an average of 0.57 percentage points since 1994. The following week, it has underperformed by an average of 0.17 percentage points. The week after that, it has outperformed. And so on.