da curve, boss
And what idiot set 90/80/70 as some sort of "standard" percentile for the A/B/C grade?
It took me a surprising long time to realise that a lot of students though I added an arbitary percentage to their numerical grades to "set the curve"... I suppose what is done can be considered functionally equivalent to doing so, except in so far as it is not.
The person setting gradable tasks and who does the grading controls the mean and range of grades, and to some extent the shape of the curve, although it is very hard to force a curve for a large class far from either a single or a double gaussian.
Personally I feel a mean percentage grade around 60% is good, with an A represented by maybe 70% or higher, though for psychological reasons it is near impossible to actually set curves that low in actual classes.
Bunching grades in a narrow percentile range makes individual grades vulnerable to random errors and single stupid mistakes and is a degraded indicator of true performance.