Well, one consideration is that the recruitment centers aren't the same in both regions. Yes, technically they're the "same" in terms of cost and size. But if we're talking about whether it makes sense, one would think design, material, space, maintenance, would differ. You're talking about different environments here, where people have different requirements to keep the region actually working, different exposures to the buildings. For example, in a rural region you might have problems with lots of animals relieving themselves next to the RC, or maybe those tenacious ivy plants keep climbing up the walls. In a city, you instead have those darn vandals thinking they're cool by inscribing their names on the walls using a hammer and chisel, or homeless commoners relieving themselves next to the RC.
So...why does it work in a rural and not a city? Because they are inherently different, even if not in terms of game mechanics. Game mechanics just uses the population, so it seems. And it makes sense that if you can fill up a rural region with 5000 people, that means you have exactly enough, or perhaps slightly more, people who actually can maintain RCs and such buildings. In a city, you get 5000 people, most of whom must fill in the bare minimum to keep the city running. The rest? Who knows what they are. You wouldn't trust a barber to fix the walls of your RC, would you? That's the point I think. Rather than do the rather difficult task of tracking down just what each peasant in your city does, it uses probabilities. At that percentage of total population, then it is likely that you have enough masons, builders, architects, thatchers, whatever, to maintain your infrastructure. Before that, there is a low probability of having the right people, and so the game just considers it as you don't have them, and you suffer decay.
