Reinforcement decrease

I was playing a normal world map game with turkey and suddenly reinforcements on Ankara reduced from 9 to 4 at turn 18ish. Is this a game mechanic of some sort or a bug?

PD: I'd have taken a screen but its kind of pointless I guess

It's because there were many battles in that city and on each battle population is reduced so that is probably why you have 4.

I think the bombers do the most Population causalities.

Less population (usually due to use of bombers) decreases reinforcement.

Ok, thanks!