9 and 10 reinforcements

I played some map and after 18 turns in my city i get 9 reinforcements but it should get 8.
and after 29 it gets 9.
How this work?

Sometimes i don't get reinforcements on reinforce week.

Its because of population casualties.

Every week the population in your cities grows. and after some time you have so many new people in your city. that you get an extry reinforcement.

But It can also go the other way. If you get attack by units what destroy your cities (bomber,....) then you lose some population and might have less reinforcement than before.