I've looked at this Mandrill knowledgebase article over and over and cannot see any obvious explanation of what the difference is between Global Merge Vars and Merge Vars, and when I should use global vs normal.
Both global_merge_vars and merge_vars are declared within the 'message' object (as shown here in the Mandrill API), so it's not like you're declaring global_merge_vars outside the message object and then including multiple merge_vars.
Also - is there any difference between how these are declared in the template itself or do they all just use *|MERGE_VAR_NAME|* format?