This is what MFL has to say on the subject:
Every week, a couple of days after all of the games are final and your fantasy scores have been calculated, the Elias Sports Bureau (which is the official statistician for the NFL) reviews all of the games on tape and sometimes issues official stat corrections if needed. For example, one of the most common stat changes is when a tackle or sack is awarded to a defensive player after reviewing the game film. This can happen because the wrong player's number was written down as part of the official scoring during the game, or because it was initially recorded as a solo tackle, but after further review, they determine that a second defensive player deserves an assist on the tackle or sack. The bottom line is that there are stat corrections every week, but you never know how much they will impact your league until after they are made official. For what it's worth, stat corrections are much more common on the defensive side of the ball, especially for leagues with IDPs (Individual Defensive Players) that award points for sacks and tackles. Significant changes don't happen nearly as often for offensive players.
Unfortunately, there is not a deadline or schedule for official stat changes, so they can technically be made at anytime, even weeks after the games have been played. Luckily, most official changes are made within a few days. Therefore, we publish all of the changes for the current week on Thursday mornings. Any changes that are made after Thursday morning are not tracked or applied by our system. That way, even if you choose to apply the changes to your league automatically, you can be assured that the scores for the current week will not change after the next week starts. As an example of how late an offiical stat change can occur, refer to this article on NFL.com, where they discuss the Roethlisberger Touchdown change, but they also mention a sack from Week 1 that was just changed here in Week 5
