Why Does She Grade Like This???

In my AP Calculus AB course, my lovable teacher does homework grades in a weird and crazy way. You are given an automatic A+ on Homework and if you don't do an assignment, you lolose 2 points. You only have 20 points a quarter. So if you do the quick math, (go figure) 20/2=10. You get 10 chances to fuck up before it turns into a zero. If you equate this into percents, you get the following
Assignments missed       Homework Grade
               1                                  90%    A-
               2                                  80%    B-
               3                                  70%    C-
               4                                  60%    D-
             5-10                             50-0%    FIs this a fair system? Yes if you do every homework assignment. But let's factor in that we are all human. You can forget up to 2 homework assignments before you start to lose ground on your grade in the class. This is a BIG GRADE too.  Now I ask you, my humble followers, What system of grading do you think is unfair?