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

For example, the following course formats are proposed at the same department of the same university (the list is not complete):

• 12 credits (48+72 hours)
• 9 credits (24+12 hours)
• 9 credits (24+0 hours)
• 6 credits (24+36 hours)
• 0 credits (one lecture; those are not really courses, but are 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. "x+y" hours means x hours of lectures and y hours of practical sessions.

Intuitively, one would suppose that the number of credits should be somehow proportional to the number of hours, but this is not the case here.

The university is in France.

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