When adjusting the default Grade point setting, it only seems to impact Assignments and the Gradebook Course total.

To replicate
1. Access Site Administration
2. Click Grades and select General settings
3. Edit Grade point default - let's say to 10
4. Save changes

To Test in an Assignment:
1. Edit an Assignment Settings
2. Expand the Grade section
3. Confirm Maximum grade =sGrade point default (see above)
this works.

To Test in a Forum
1. Edit a Forum Settings
2. Expand the Whole forum grading Grade section
3. Change Grade type to Point
4. Confirm Maximum grade = Grade point default (see above)
Grade General Settings do not adjust max grade for forums.

To test Gradebook changes (grade items and course total), in a newly created course
1. Access the course gradebook
2. Click the Grade menu and select Gradebook set-up
3. Add a grade item and confirm Maximum grade setting
4. View Course total

The grade item max adjusts as does the Course total.

It would make sense that this setting changes a Grade item, Assignments, Forums, and other core Moodle learning activities. Not sure why adjusting this setting would also adjust the default value for the Course total aggregation.