How can I determine which activities are included in the "Days Since Last Activity" field default? (e.g., I created a Smart List with Days Since Last Activity being >180 days, however, the final list included some lead records that had opened an email that we sent 2 weeks ago)
That field (Days Since Last Activity) looks to be a custom field. This would mean that the filter is looking solely at the field value, and not act any actual activities. For those leads you don't think should be on the list, what value do they have in this field? Is it possible the field didn't get updated when it should have?
You could, instead, use several filters if you are looking for people with any (or no) activity:
and the Not filters, if you want people who haven't done anything.