They have more complete records from the Sacred Plaques. Depending on what source you read they either had very few plaques lots or none at all. They were not surprised by the coming of the Skaven or Sotek.
They have more complete records because they have far less contact with non-Lizardmen and get in fewer fights with other races.
Depending on how you interpret fluff. Slann in the Southlands and Lustria either have zero or very little telepathic contact so the Southlands Slann make their decisions in a vaccuum. On one occasion this led to a massive fleet of ships being hit by two freak storms a few days apart since both Lustrian and Southlands Slann orchestrated their own storm in accordance with a prophecy and made slightly different calculations as to timing.
I'm pretty sure the Southlands doesn't have any living Second Generation Slann.
They have comparably fewer Saurus. In 6th edition they had a different army list with the following major differences:
-Skink Cavalry instead of Saurus Cavalry
-Skinks could have short bows instead of javelins and shields or blowpipes
-Saurus Warriors were Special, Scar Veterans were Lords, Oldbloods were not available, Temple Guard could not be taken without Slann
-Back when choices were limited by numbers instead of % points, a Southlands army could take 2 Skink Characters as a Lord choice
They never had a major fight with Araby and even hired a team of Araby explorers to help them find water in the desert so they could attack a Nehekaran Tomb King who had stolen a mummified Slann.
Tichi Huichi's Raiders, the Lizardmen Regiment of Renown were from the Southlands. They have traveled far working as mercenaries for many different races recovering many lost artifacts of the Lizardmen (presumably some of them are from Lustria).
