The electoral system in Catalonia (or Spain) is not exactly proportional. There are several biases, but the most important one is that the electors are divided

The electoral system in Catalonia (or Spain) is not exactly proportional. There are several biases, but the most important one is that the electors are divided in several districts (Barcelona, Girona, Lleida and Tarragona), and the number of seats assigned to each district is not exactly proportional (it's 2 + the proportional part among the remaining seats). This causes a bias towards the parties that get the most votes in the less populated (typically rural) districts, and those are the regions which have the most support for independence.