Catan setup order different than playing order
I think you have got how you work out start player wrong.
You said &quot;the second highest number goes second and so on.&quot; The rules do not state this.
Looking at rules here it says :-

Each player rolls both dice. The player who rolls highest is the
starting player and begins.  The starting player places a settlement
on an open intersection of his choice. He places a road adjacent to
this settlement.  The other players then follow clockwise. Everyone
places 1 settlement and 1 adjoining road.

So whoever rolls highest is the start player.  All other dice rolls are irrelevant.
As an example for you imagine for players sat clockwise around a table A, B, C and D.
A rolls 11, B rolls 2, C rolls 8 and D rolls 4.
Player A rolled highest and placed the first settlement.  Player B is next clockwise and places next.  The fact Player B rolled the lowest number is irrelevant as the dice roll was for start player and not the turn order.  The player C, the D.  Then D again and back anticlockwise, C, B and finally A.
Settlers of Catan is not a game where the turn order changed during the game (such as say Caylus or Power Grid).  So you only need to randomly select a start player.  You do not need to randomly select the whole player order.
In my mind the point of selecting starting locations ABCDDCBA is to try and minimise disadvantage for selecting before other players by letter the player placing last get first pick of second settlement location.   If they placed them ABCDABCD then player D would be at a huge disadvantage.