I answered that.
Because they CHOSE to both work on the door as a team, at the same time, instead of taking turns individually and thus use the task chain rules. Which means that the helper check modifies the leader check and does not complete the task.
Per the text you quoted, they indeed had the option of taking turns. Usually, forcing a door is a task you can keep attempting.
I just realised you might also have misunderstood the respective target numbers. Erik is rolling against the task difficulty (presumably 8+, but it might be higher) - Kathya is not. She is rolling a simple STR check (target number 8+), and if she rolls a 8 gives him a +1, or a +2 on a 9-12.
Does that help?