Why isn't GPA a simple average of grades?
GPA is a weighted average — a course worth 4 credit hours should affect your GPA more than a course worth 2 credit hours, even with the same grade. That's why you need each course's credit hours, not just the letter grade.
Semester GPA vs. Cumulative GPA
Semester GPA covers just the courses in one term. Cumulative GPA covers everything since you started, factoring in all credit hours from every term combined.
Grading scales vary by school
Some schools use a 4.0 scale, others 5.0, and the exact point value for each letter grade can differ. Always double-check against your institution's official scale before relying on the result.