Marking papers
This is standard practice. Remember, these people are marking papers, not correcting papers. The difference may be subtle, but important.
A teacher / lecturer / TA will correct essays / test etc during the course of an academic year. They will tell you where you went wrong, give suggestions and recommend reading x,y or z.
Exams don't get corrected, they get marked. The marker will have defined parameters in which to work, looking for the depth and breadth of knowledge depending on strict criteria. In this respect, it is quite widespread that anyone with experience of marking one type of paper, is capable of marking another within the same broad field. Naturally, science subjects are the exception, where the content is very specialised and the notaqtions different.