As many people have explained before:
Because all 3 teams were tied in their head to head, to decide how tie breakers happened, the team with that won their in the least amount of time would be the "Final Boss" of the tie breakers, and that was the Gygabyte Marines. This instantly means that no matter what, Gygabyte were not ending 4th in the group. IMT lost the first tie breaker and locked them as 4th in their group, with GAM and FNC playing for 2nd and 3rd.
This is not a but. It's just you not reading the rules or even watching the stream when they explain how the tie breakers work.