How do they choose first map pick?

Yesterday Gambit and Envy got to pick the maps.
How does Riot choose who gets to pick the first and 3rd map?
I thought G2 would've got to pick first against Gambit because they won their previous match 2-0 while Gambit won 2-1. or because G2 was first seed and Gambit wasn't.
But actually Gambit got to pick.