Even excellent students can fail math if they don't think things through.
I scored 13 out of 14 points on the final exam, which was in the ЕГЭ format (13-14 points is a top grade), but I received a 4 because in my draft, I initially solved problem B5 using the substitution method, and then I realized how it was solved and wrote down the solution. They marked it down and wrote "no solution," even though I had the correct answer and two solutions (they checked our drafts). They penalized me for writing the following in my draft:
"Solved by trial and error
*solution*
Scientific justification: *another solution*