I can't say that your concern is not 100% valid. Summary (Aggregate) values for things like Inventory or $ balance varies depending on how the business uses these value and it could be Beginning Balance or Closing Balance. In other cases, summary methods such as average or a function are more appropriate..
This is really not an Anaplan "thing", in a client situation you would have to explicitly ask how the aggregate summary value is used to determine the most suited Summary method.
In this case, the course chose to use the Closing Balance summary method. I believe it could have been chosen as the beginning balance as well.
Best of luck on your learning journey
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I've now learnt to ask such questions to the academy directly and not post here.
It was more of a question behind the logic of the course content and not a direct Anaplan question. The summary/time summary functionality is great and I agree that in a Client situation one would just ask the Client what they needed to use the field for.
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