Borders moved before your family did
A family memory of nationality and a legal claim to citizenship are not the same thing - and nowhere is that gap wider than in the lands that changed hands after 1918.
Ask someone whose family left Central or Eastern Europe a century ago what nationality their ancestor was, and you will usually get the name of an empire, not a country: Austrian, Russian, Ottoman. Those empires no longer exist, and the citizenship law that matters today was written by the states that replaced them - states that often drew their borders through the middle of the empire’s old subjects.
Galicia is the clearest example. Someone born there in the 1880s was, on paper, a subject of Austria-Hungary. When the empire dissolved in 1918, that fact did not carry forward as Austrian citizenship. Instead, each successor state assigned citizenship to the people living within its new borders - mostly Poland, in Galicia’s case, later the Soviet Union for the eastern portion. "We were Austrian" is a true memory of empire and a false claim to Austrian nationality law, held at the same time.
The same pattern repeats across the region: the Kresy, Poland’s interwar eastern territories now in Ukraine, Belarus and Lithuania; the redrawing of the Baltic states; the breakup of Czechoslovakia and Yugoslavia decades later. In each case, the country an ancestor left is not necessarily the country whose citizenship law now applies to their descendants.
This matters practically, not just historically. Most people checking their eligibility for a foreign citizenship start with the country their family told them about, and stop if that door looks shut. But a shut door in one country can mean an open one in its neighbour - the same ancestor, the same facts, checked against the right state’s law instead of the one your family happened to remember.