I was playing on a small map with 2 AI's. We each had around 30 population. After I took over the two cities of one faction, suddendly the news appeard that I was near a military victory. When I checked the percentage, it showed 88%. Afterwards I sent some planes to the remaining faction and counted our population. He had 33 and I had 54. In my opinion thats quite less than 75% of the total population

I tried to fast forward the game multiple turns but there never appeard the winning screen (sometime later the economic victory news came and soon afterwards the economic winnning window).
My guess why this bug happened: Before I eliminated the first faction we traded a city back and forth a few times (I think 3 or 4 times) but it was only a small city with 2-5 population. Perhaps the gain in faction population gets added after a successful take over of a city but when you loose a city, it doesn't get substracted.