For a good crosstable, duplicate games should be marked for deletion, and Player, Site and Event names should be spelt consistently. See Database Maintenance for more information.
Clicking on a Game Result shows a menu from which one may Browse, Load or Merge a game.
From the menu you'll be able to edit the tournament details, export the table as Text, LaTex or HTML, choose the Sort Criteria, and select various other options. Right-clicking the crosstable displays the menu.
When sorting by score, players with equal scores are ordered by their tie-break. These are Sonneborn-Berger for All-Play-All, and Bucholz for Swiss. Optionally, Head-to-Head and Total-Wins tie-breaks (in that order), can also be used.
To calculate ELO Performance ratings and ratings changes, Scid uses algorithms from the FIDE handbook article 172 , or older versions of this article. Some related discussion can be found here.
Compared to some sources, Scid's Ratings Changes have small discrepencies, due to taking average opponent scores. Persons interested in authoritatively updating Scid's statistics will find the relevant code in Crosstable::RatingChange.