I randomly started checking other CompSci courses on that site and they all had a similarly high average grade, in some cases even higher (A instead of a B+).
Which are the hard courses at Berkeley in CompSci using the site you linked?
The hardest CS undergraduate course offered is probably 189: Introduction to Machine Learning. Nonetheless that class has a B+ average; so I wouldn’t say difficulty of class correlates with a low grade average. There are others like 170 and 162 which you can check out.
If my memory holds, there’s a policy that class averages should be around 3.0-3.3 (B/B+).
Which are the hard courses at Berkeley in CompSci using the site you linked?