No, you just don't understand the concept.
This example = FAIL
First of all if this was the case the professor would hand out a course outline of this which is essentially a contract for the course. If what is stated is true by the OP then nobody can fail if all test were averaged B,D, F and weighted equally. That does not equal an F grade.
The test scores can be averaged out, but the grading system remains constant.
Assume three students. and a scale of 90-100=A, 80-89=B, 70-79=C. 60-69=D. Below 60 = Fail
Scores are 100-A, 80-B, and 60-C. That's' 240. Divided by three gives an average of 80, or a B. This works out well for the guy that barely passed, neutral for the middle guy but the A student took a big hit.
Let's assume that the A student kept studying anyway, the guy in the middle slacked off and the loser said "Hey, I like this" and stopped studying entirely.
Let's try different numbers now; 100-A, 70-C and 40-fail. That's 210. Divide that by three and we have an average 70, barely a C. Again, the only loser is the A student.
Now the brain says fluck it, why should I bother, and stops studying.
New grades; 80-B, 60-C and 40-fail. That's 160, divide by three and we have 53.
Congratulations! Socialism has killed the class average.
Don't they teach these concepts where you're from?