I just think that certain aspects of a film contain more importance to the overall quality then others.

For example, I give direction a score out of 20 because I feel like a film could not have good quality without good direction, so that score is a large percentage of the total score, while I give music a score out of 5 because I feel as though a film could still be good even if it had shitty music.

The categories are scored out of the set amounts because they reflect what I feel is their importance to the overall quality of the film.