Math Teachers.

So, throughout the years i've had many different math teachers. Some have been good and some have been bad, but they never grade consistently! Some of them you get the right answer you get the problem right, others if you write one thing down that isn't right or you don't write every single thing you did to find the answer, its wrong. So what if i didn't rewrite the entire problem after every step, i did the work right and got the answer and still showed all my work. But no, just because i didn't write one number down i dont get credit for the entire problem or even assignment! Why?