A large proportion of the population of Polesie claimed to be "tutejszy" (local) rather than Polish or other as their nationality.
And there are different theories as to why this is but it appears to be down to a lack of trust between the census takers and respondents who were of different ethnicities.
An ethnic Ukrainian or ethnic Belarusian with nationalist aspirations would be less inclined to reveal their true identity to a Polish census taker. Vague responses would have been a tactic to give the census takers misleading information while quickly sending them away. Using the vague term could have been a form of resistance or done out of unfounded fear of consequences by having information collected on them or even fear of reprisals within their own communities by assisting Polish census takers. Indeed, Ukrainian and Belarusian nationalists would never have been successful if they didn't have the "tutejszy" on their sides. Obviously the "tutejszy" were only such when dealing with Polish officialdom and not amongst their own when the Poles were away.
Another thought is that the use of the term may have been allowed by Polish authorities during that time period as a means to dissuade anti-Polish nationalism from spreading by showing in census figures that ethnic Ukrainians and ethnic Belarusians were not a large collective respectively. Being told their numbers were small would discourage open hostility because they would believe they had no strength in numbers.
Either way it didn't work in the end. The term never curtailed the nationalist movements from succeeding in achieving independent statehood nor did it dissipate Ukrainian and Belarusian enmity towards Poles.