They might've bought into the lie that Jews don't believe in Jesus. Thus, they may consider themselves Polish Catholics and not Ashkenazi Jewish Catholics. Also, as you said, someone was likely Jewish and hid it. My granddad went from utter denial and trying to connect us to Stefan Czarniecki to "If we had any Jewish blood, I don't know about it." My dad also used the classic Jesse Straus excuse in relation to our family:
As to our 'Jewish roots', I have talked to a number of people and there is only one situation of someone on the Rusnak side of the family being Jewish and that is questionable. Please remember that Judaism is a religion and not a nationality and that in your extended family, you probably have scores of Christian denominations and sub-denominations represented as well as other religions.
That was a while back, and I think that he is now beginning to begrudgingly concede that we do have Jewish roots. Meanwhile, I just realized that he used Straus' exact wording. Incidentally, he told me that the member in question was Henry Feldman, and Henry's wife (Elizabeth Peregrin Feldman) happens to be a maternal Fosko Rusnak. This, thus, makes her a maternal Levite (The Foczkos were Levi'im
from Warszawa, Radom, and £ódż; and our branch became Anusim
and immigrated to Slovakian Hungary. Istvan Foczko married Johanna Hanzokova whose mother was a Lazarova, so we may also be of kohein
descent and descended from Ele'azar HaKohen ben Aharon
on that side.
(As for the Rusznaks, they were Levi'im from Kosice.).