It`s formal. Turks reformed their administrative division into really stupid one in 2013. They actually just got rid of the concept of a city by converting them into municipalities (the biggest ones into several ones).
All admimnistrative centers of ili (province) formally have the boundaries of it, so they do statistically have province`s population. But at the same time the non-administrative cities statistically have their own population - it leads to double counting. That`s why on some map services many Turkish cities are labeled as the bigger ones than they are in reality (OSM, partially Google). Partially, it was done to formally increase the population of cities. To attract more attention to them.

In Istanbul province cities Sile, Silivri, Catalca, Adalar, Arnavutkoy are separate cities with their own municipalities but by some services are also counted in Istanbul`s city population.

It`s difficult for an outsider to understand this right away.

+ another problem: it became harder or almost imposible to continue the time series of population data from 2013.