If you look at the questionniares you will see that the Universe of these questions that these changed across waves, but also all who should have been asked were not asked - that is why the Universe was changed later.
In Wave 1, all those not born in the UK were asked this question. In Waves 2-4 all new entrants not born in the UK were also asked this question. Those who said they didn't have a citizenship, could have acquired since the first interview. So, from onwards Wave 5, the universe was changed - in addition to non-UK born new entrants, continuing members who were not a citizen was asked again. Please note that since the 1980s, being born in the UK does not result in automatic UK citizenship. So, it is possible that some of the UK born are not citizens. Here is more information, https://www.gov.uk/apply-citizenship-born-uk/uk-until-10
Additional variables, such as year & country of birth, country of birth of parents, year parents came to the UK are also available in the data. These may help in identifying UK citizens among the UK born who were not asked this question.
Hope this helps.
Best wishes,
Understanding Society User Support Team