When you place a summary field defined in a related table on a layout, the summary field will compute an aggregate value (total) based on the set of records in the related table that are linked to the current record in the layout--which would probably be a single record in Accounts in this case.
One solution is to define a new relationship to a different Tutorial: What are Table Occurrences? of accounts that matches to all account records (such as by using X instead of = as the relationship operator) and then select your summary field from this occurrence of accounts. Since the relationship matches to all records, you then get a total based on all records.
As usual, you provide a clear, concise explanation. Thank you!