Assume, for instance, that one connects percentage grades with letter grades by having 90 mark the dividing line between A and B, and 80 for B/C, 70 for C/D, 60 for D/F. (I don't know that MNPS systematically uses these dividing lines but they are likely in the ballpark of how most people think about grades on a 100% scale.) Giving a zero on something throws into the student's average a number for failing performance that is lower than the distance between C and D by a factor of five. It makes no sense to do that. One has to earn 150 on another assignment just to balance the the failure to achieve an average of 75.

Think about the 4.0 grade point average (GPA) scale common in higher education and some secondary schools. That scale makes sense because grade values of 4 (for an A), 3(B), 2(C), 1(D), and 0 are arithmetically equidistant, and because the number assigned for failing performance and non-performance is the same: zero. In linear mathematical terms, giving a student a zero on a 100-point scale where 60 is otherwise the D/F dividing line (so let's call 55 a straight-up F) is the equivalent of giving a student graded on a 4.0 scale a grade point value of -5.5 and then averaging it in with all the other scores that range from 0.0 to 4.0.

The use of 100-point grading scales has always been idiotic for this reason, unless the teacher curves or concatenates the bottom end of the available range so that the lowest possible score (somewhere in the 50s, presumably) is in sync with the magnitude of steps across the overall range of grades. MNPS' "let's bottom out at 50" policy accomplishes this, making the system smarter, not dumber.