Because most private schools tell there students the coursework answers resulting in majority of students getting 49/50 or 50/50 so grade boundaries are ridiculously high. Meaning the rest of us who dont cheat and are not given any help get 43/44/45/46 out of 50 which is the range of E-C grades...
A teacher can tell their class the entire answer (or give obviously massive hints) so they are guranteed more or less very good answers - means schools / teachers which do it properly will more likely face worse grades + boundaries
Not really coursework but I got a D in my ISA and an A overall. I think most schools just 'help' more than others so grade boundaries are ridiculously high