Don't know exactly your case. Sometimes, a teacher may wish to build the foundation of a student, in order to him/her to be prepared for a more advanced level. May stuck for a while before obvious improvement. On the other hand, it could be a teacher's mis-judgment like you said.
A teacher will usually plan ahead so that this is enough time for the kid to get prepared so that he/she will get likely get at least a high merit under normal circumstances...
External factors during the exam could make the kid perform at sub-par level. Taste of the examiner and the different thoughts of interpreting the exam pieces between the examiner and the teacher could also affect. Hopefully his/her fundamental skills and shapes could remain, hence getting a pass at least...