Suppose a team takes the opening map and wins the second 13-11. It wins the best-of-three series 2-0. A -1.5 map handicap wins, but a -2.5 round handicap on map two falls short because that margin was two rounds.
Both sides' rounds count. At 13-11 there were 24 rounds, enough for over 23.5 on that map.
The third map is unnecessary in a best of three. Two maps were played, which settles under 2.5 maps.
More sports: Valorant, Dota 2, League of Legends.