Is a rough percentage of your # of military units (no settlers or workers) divided by your # of citizens. It represents the rough % of your citizens that are military. Infact, if I remember correctly, this isn't even a direct correlation, but actually the above frmula multiplied by 10 and shown in years.
Bamspeedy did something in regards to demographics, but the search is disabled so could not find.
An example: You have 100 military units and France has 100 military units. However, your total population is less than France. On the demographics screen, you would have a "better" Military Service because a greater portion of your overall citizenry is a military unit.
Hope that helps.
My 2 cents