I like to think of the assignment groups as gradebooks within gradebooks, and using weighted grades means your course total points is 1 (or 100%).
So, for example, in your situation if the student got 90/100 for dialogues, that is 90%. 90% of 40 is 36. That's .36 points towards the final grade.
If a student got 150/300, 50% of Creative projects, 50% of 25 is 12.5, so that is .125 points towards the final grade.
.125+.36 = .485 towards the final grade.
So to answer your question, "what is the difference than if the assignments in both groups added up to 100 points?" the only difference is you've chosen to group those assignments to make them add up to a percentage of their final grade.
Most folks who use weighted grades use it to ensure that different types of assignments effect (weigh) differently to effect the final grade. For example, in higher ed. we often want assessments to weigh much more than home work or assignments. So if I would make assessments weigh 80% and homework 20%, I'm guaranteed that no matter how many points are given for homework, it will only be worth 20% of the final grade. So, if a student did no homework assignments in the course, the best they could ever get in the course, assuming they received perfect scores on the assessments, would be an 80%.
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