This is a pretty picture, and the traditional Roman and the Byzantine rites obviously share the same faith (and are equally medieval in evolution), but here is some historical background.
Ruthenian (western Ukrainian) Catholic mountaineers in Poland were heroes fighting but losing a guerrilla war with the Polish Communists. The Poles, who never liked them simply because they're not Polish, dispersed them, ethnically cleansing the Ruthenian villages, either deporting the people to the USSR or moving them elsewhere in Poland. The Ruthenians' Greek Catholic churches were given to the majority Latin Church in Poland.
Doctrinally and liturgically sound Catholics aren't always right. Witness the Chornock schism in America.
One of the reasons I go to a Byzantine Catholic church one Sunday a month and worship there exactly like the Orthodox (bowing, not kneeling, no filioque, and all).
Без числа согреших, Господи; прости мя.
Someone who was at this Mass clarified that they were having a retreat nearby and needed somewhere to have their Masses. So the priest at the nearby Byzantine church, the closest church to where they were having their retreat, let them have their Masses there for the duration of the aforementioned retreat.Thank you. But such churches and mixtures are in southern and eastern Poland for the reason I give.