From the FIDE handbook

8.22 If he scores 50%, then Ru = Ra

8.23 If he scores more than 50%, then Ru = Ra + 20 for each half point scored over 50%

8.22 If he scores less than 50% in a Swiss or team tournament: Ru = Ra + dp

8.25 If he scores less than 50% in a round-robin: Ru = Ra + dp x n/(n+1).

What's the justification for rating calculation rules for unrated players being different between round-robin and Swiss tournaments.

See also What is the justification for the calculations of FIDE previously unrated player not being balanced?

From the FIDE handbook

8.22 If he scores 50%, then Ru = Ra

8.23 If he scores more than 50%, then Ru = Ra + 20 for each half point scored over 50%

8.22 If he scores less than 50% in a Swiss or team tournament: Ru = Ra + dp

8.25 If he scores less than 50% in a round-robin: Ru = Ra + dp x n/(n+1).

What's the justification for rating calculation rules for unrated players being different between round-robin and Swiss tournaments.

See also What is the justification for the calculations of FIDE previously unrated player not being balanced?

As of March 1, 2024 the FIDE initial rating calculations are now defined in a consistent way.