I think it’s a mentality thing. Real Madrid tend to get complacent against smaller clubs in la liga but rise to the occasion on big UCL nights. Barca have had a tendency to succumb to the pressure on return legs, but will refuse to drop points in la liga for long periods of time. They very rarely have ”off days” againt middle and bottom of the table teams. Also, I don’t know if this is true, but i hear Spanish teams love beating madrid, so when it’s time to face them they treat it like a final.