In addition to the secondary, they also used a point system for interview invitation. There are three categories: mcat/gpa, personal statement/secondary/lor, extracurriculars (research, leadership, etc.).
Each section is worth 5 points for a total of 15 points. To be rewarded with an interview, one must get 11+ points.
If you are the average joe and get 3's on each section, then you won't get an interview. Thus, it is important to write a good secondary.
My sources are both from a former adcom and a current adcom at uci.