If you are picking a playing fixture with 8 teams playing each other, there are more than 28 ways any individual Round can structure up.
28 would be the number of picking 2 teams at random to play each other.
Start with team 1. They have 7 potential opponents. That leaves 6 teams. Pick any of the 6 at random, and they have 5 potential opponents.
That leaves 4 teams.
These 4 teams can only make 3 structures:
A v B, C v D
A v C, B v D
A v D, B v C
7 x 5 x 3 = 105 ways to structure the Round.
Number of ways 1 to 4 can structure to play 5 to 8.
Team 1 can have 4 opponents.
Team 2 can have 3
Team 3 can have 2
Leaving Team 4 with 1.
4 x 3 x 2 x 1 = 24.
24/105 = 22.9%
Happy to have myself corrected, if I've got this wrong.