Query & Dashboard Output Clarifications

I've logged a few tickets recently about using queries for data reporting to our senior team (as has been the best option until now). I have found that the filtering options/way data is treated is slightly different between the query and dashboard). My query was differing headcount.

Could we have something that clearly details what the dashboard filtering is actually doing. It is different to a query pulling the same information. The support agent explained the different calculations for my headcount issue but I wonder if there are more differences we all need to understand.