I imagine the cutoffs are based on the percentage of students able to achieve a certain score. I wouldn't make a whole lot of sense to have the score for "5" set at 90% if only 1 in 200 students got above a 90%. For example, the national average on most biology written responses is well below 50% of the points, and considering that statistic, I think the lower cutoffs are quite reasonable.
I imagine the cutoffs are based on the percentage of students able to achieve a certain score. I wouldn't make a whole lot of sense to have the score for "5" set at 90% if only 1 in 200 students got above a 90%. For example, the national average on most biology written responses is well below 50% of the points, and considering that statistic, I think the lower cutoffs are quite reasonable.