Why can courses at the same department have an apparent disproportionality between credits and hours?

For example at my French university my department offers a 12 credit course with 48 hours of lectures and 72 hours of practical work, a 9 credit course with 24 hours of lectures and 12 hours of practical work, a different 9 credit course with 24 hours of lectures and no practical work, a 6 credit course with 24 hours of lectures and 36 hours of practical work. There are also 0 credit courses with 1 hour of lecture, but these are not really courses and are just included in the course catalog as bonus content to make up a bit for courses not offered in the current year. All of the courses I refer to are Master's level.

How are the number of credits calculated? Intuitively, one would suppose that the number of credits should be somehow proportional to the number of hours, but it is clearly not simply: a x # of lecture hours + b x # of practical hours.

Question: What is the relationship between credits and course hours?