My understanding of index funds like QQQ and SPY, is that they are built to mirror the index movements from which they are created. And as is typically the case, the index funds move with the index both in direction and magnitude. But there have been a number of times that the index and a corresponding index fund have moved quite differently. The most recent large difference that I noticed was a couple of days ago on the 24th of December where QQQ was down ~2% and the Nasdaq was down 5%.
Why was there such a difference in the movement of QQQ and Nasdaq in this case, and why does this occasionally happen?