Uneven group sizes

example: 3 Rounds 13x13 II (15+ players)

we had 21 players in a 3 round simultaneous mcmahon. you’d expect to get 3 even groups with 7 players, instead we got 8+8+5.
this is inherently unfair to the group with 5 players, since they cannot get more than 4 points in this first round. why doesn’t the algorithm try to create equally sized groups?