This may be in some other part of the Wiki but I can't find it - why do I have a few hundred total population but only 25-30 available population? Where are the other 300 people?
Your total population is the sum of all the population produced by your residential buildings.
This population is required by different other buildings like goods, military, production etc. What available population left means those people who are free/not engaged in these buildings.
What do you think?