Events are not always received in the order in which they completed and are rated, some TDs get their events to us faster than others (Most, but not all, events are submitted online, and most TDs have their events to us within 2-3 days of when it ends.) In addition, events sometimes need to be corrected, for example if a result was mis-recorded.
Both of these can cause changes in the post-event ratings for an event (and the pre-event ratings for subsequent events.) Rerating is designed to address both issues. Rerating recomputes the ratings from events after reordering them and taking into account any corrections, in other words, what they should have been if every event had been received in the order in which it is rated and if every event was received error-free.
We currently re-rate events on Tuesdays, though it can take up to 2 days for rerated events to be posted on MSA. We also do a full rerate (back to 2004) before the creation of a new monthly rating list.