Skill based Match making in apex legends doesn't go off of the party leaders SBMM, it goes off of the highest skilled player in said party. Similar to how ranked works where if there's a gold and a diamond playing together, the SBMM works off of the diamond players lobbies. Or your in a low population queue where the lobby skill gap is opened more due to a low influx of players, playing at that said time
TLDR
Their current skill rating is close enough to yours or your teamate for the matchmaker to put you in the same lobby
You‘re in a high-skilled lobby in a premade squad with a player who is higher ranked than yourself
You’re playing in a low population queue (Continous window Matchmaking)
full breakdown here : https://www.ea.com/en/games/apex-legends/apex-legends/news/matchmaking-update-0924