I don't want to comment on the figures you have posted because I'm not sure how valid they are. The Zhitomir and Vinnitsia ROC vs UOC-KP adherance figures seem to be off - but I would not bet my borsch money on that

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The issue is confusing only because we are speaking about eparchies which have found themselves in different countries and empires. I should have made things more clear.
UKRAINE:
After the Soviet invasion of 1945, in Ukraine, some Greek Catholics whent to the Latin rite church to avoid becoming Russian Orthodox. After independance in 1991, some returned to the Greek Catholic Church and others remained within the Latin Rite. The Roman Catholic Church put their adherance number at 700 - 800,000 in 1991, but Ukrainian Government statistics put it at about 200,000. Is the difference the number of people who returned to the Greek Catholic faith ? I don't know for sure

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The high number of RC parishes (but not adherants) you see in areas such areas as Vinnitsia, Zhitomir, and Lviv are remnants of the Polish-Lithuanian Commonwealth. Small numbers of adherants survived in areas where they had been more numerous before (ie: pre-Soviet rule). As I mentioned earlier, I have visited famillies in the Rava Ruska area northwest of Lviv were parish churches have remained with as few as 4 famillies, but because they get money from Poland (who collect from Catholic charities) they were able to be sustained.
Vinnitsia and Zhitomir (unlike Lviv) had few Greek Catholics because they where part of the Russian Empire for a period of time and then the Soviet Union from the beginning. Greek Catholics where not tolerated in either. After 1991, the Greek Catholics who appeared were those who moved there from the far western Ukrainian lands. I doubt if any where 'Closet Greek Catholics' hiding in the Latin Church.
SLOVAKIA
Lviv and SubCarpathian Rus were part of the Austro-Hungarian Empire up to World War 1. Preserving the Greek Catholic faith was always a struggle, but could be done. After WW1 the Carpatho-Rusyns endured strong Slovakization / Latinization and the Galicians strong Polonization / Latinization.
The pressure to assimmilate into the larger Latin rite Slovak Church during the interwar period was so strong that both bishops of Mukachiv and Priashiv supported the declaration of a Carpatho-Ukrainian Republic in 1939. They felt that only be joining the larger Rusyn umbrella group known as Ukranians could their Greek Catholic Church survive. They choose Monsignor Avgustin Voloshyn of the Greek Catholic Church as their president, but the Hungarians who were allied with the NAZIS put down the new government in a very brutal manner. In 1941 the Galicians proclaimed an independant Western Ukrainian Republic but had a whole 6 days before the leadership was arrested and send to concentration camps by the NAZIS. The Galicians lasted 6 days longer that Carpatho-Rusins when faced with NAZI brutality.
IMHO, the 'Closet Greek Catholics' hiding in the Latin Church were probably in the two eparchies of the Greek Catholic Church who in many ways had the most similar histories: Mukachiv and Lviv eparchies (now in Ukraine), but not Priajshiv (now in Slovakia). Priashiv eparchy is more similar to Kholm except for the fact that the Slovak Greek Catholics did not know 'forced' Russian Orthodoxy until after World War 2.
Kyiv and Chernihiv which were never part of the Polish-Lithuanian Commonwealth will have few Roman Catholic Parishes. The people that populate these parishes are either the progeny of business people who came to trade in the Russian Empire, old pre-revolution land owners or Latin Catholics who found themselves in these areas for one reason or another. There are relatively few.
POLAND:
(These adherance numbers are not found on the table you posted earlier).
The Khelm UGCC eparchy which is now in Poland, and which Fr. Peter represents, was part of Poland in the interwar period. Polonization (Latinization) occured in the interwar period, and then further after the war when the communists came to power. Most Greek Catholics became Latin Rite and some whent to the Polish Orthodox Church. This situation is similar to that of present day Priashiv (Slovakia) for Greek Catholics, but not Mukachiv (Ukraine).
I hope that clarifies things
(response: it's as clear as mud

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Hritzko