Media narratives are driven by trajectory.
Things get better or worse. People rise and fall. Maybe there is an upstart sensation who threatens the establishment. Maybe there is a spectacular fall from grace. Maybe there is a comeback. Regardless of the story, the direction of movement is what matters.
President Joe Biden got caught in one of those narratives: that things were going badly and people were losing confidence. Then, of course, the polls backed up that narrative, which provided a patina of proof.