Did you read the explanation near the top of the page?
"Recently rated events that have not yet been rerated into chronological order are indicated with a double asterisk (**). The post-event ratings from those events (and subsequent events) may change when they are rerated. (Events rated prior to 2004 are shown in the order in which they were originally rated.)"
Think of a double asterisk situation as an initial iteration of the ratings. When all of the events are later run through the rating equations in chronological order, the ratings may change slightly.
Such as the most recent event on this page...
http://www.uschess.org/msa/MbrDtlTnmtHst.php?14661664