I agree that it would be the most efficient way but, as we all know from this process, schools do not necessarily go down the most efficient route.
I have had a look at the information from Emanuel (as part of the acknowledgement for registration) and it says that 'There is no need to apply for academic scholarships; pupils who do very well in the entrance exams will be considered for an academic scholarship and as part of their interview and group assessment, will have an additional interview assessment.' Worth noting that (a) it does not say that people will be told in advance, and (b) it does not say it is a separate one.
You mention that the person might be from the English or Maths department. I do not see why that would need to be the case for a scholarship assessment - surely only if the scholarship is based on a specific subject (which I do not believe to be necessarily the case). Also, some DC were told (my DD included) by the interviewer their role within the school.