Why are there two grade weighting category fields when creating tests?

The one on the top is the grading category for the test itself, and the one on the bottom is for the corresponding student task to study for the test.  If you don’t plan on creating a graded study task, then as long as the Not Graded option is checked, you don’t really need to worry about the grading category for the study task because it won't be graded anyway.

See the Study For Test FAQ for an explanation on why we automatically create a study task for each test, and why some teachers choose to grade their study tasks.