I think its very common to grade on a curb, mostly because most professors don't have a natural high water mark defined (like what does "A" work mean independent of your peers?). And if they did have an idea, they probably have taught the class many times to get that idea. Especially for smaller upper division classes that are not taught at scale, this is probably not very feasible.

And yet GPA is used to rank applicants in pretty much every academic admissions process.