Your explanation of the US-inspired GPA system states that the average "is calculated by finding the average of course grades, although sometimes weightings are used to place additional value on advanced modules". But rather than being positioned as a "sometimes" element, the weighting of course credits and grades is integral to the GPA, at least in its standard iteration.
The system works as follows, given that American university courses are assigned variable credit amounts. If Mary earns an A (numerical equivalent: 4.0) for a 4-credit course, she receives 16 quality points, the product of 4.0 x 4. If, in addition, she realises a B+ (3.33) in a 3-credit course, she accumulates another 10 quality points. That 26 quality-point total is then divided by 7 - her course-credit total - giving a GPA of 3.71.
Thus the GPA's denominator - signifying the student's credit total - necessarily accords more weight to higher-credit courses.
Abbott Katz, London