That's one source of bias that is present at a specific time. Mostly you would have competing incentives. There is usually more than one agency that runs does the counting. Vital records registration, voter rolls and tax payers lists, for example are separate agencies in some countries. Not every tax payer is a voter and not everyone who was born still lives in the country. The sources are sometimes cross-referenced too. Then there is usually a place that needs to do macroeconomic forecasting and needs to have some numbers to do it's job.
This study published in Nature [0] says that rural populations in particular are typically UNDERCOUNTED (exactly like the Papa New Guinea in the OP's article), and that this happens at similar rates across poorer and wealthier countries: "no clear effect of country income on the accuracies of the five datasets can be observed."
1. Power is not only what you have but what the enemy thinks you have.
That's why somme statistics look weird. That's also why things heavily relying on demographic data need to be question. It's particularly significant when it comes to green house gas emissions for example and climate modeling.
Aren't there plenty of incentives for over expressing population numbers in many countries, specially in underdeveloped ones?
> The true population of the world, Bonesaw said, was significantly less than 1 billion people.