This is unexpected.
Not hardly - my family is primarily descended from Spanish explorers/colonists/soldiers who came over several hundred years ago (1500s-1600s), and some of the family historians have thought that some branches of the family were Jews that had converted or hidden their religion.
I don't think it's the case with us though (having hidden or converted Jews), because my family has always maintained fairly strong ties with Spain, even going so far as to occasionally go back and marry somebody from Spain - shocking as it maybe, members of my family over the centuries would go through the trouble of marrying somebody from Spain, rather than the local "mixed-bloods". I guess that makes us racist. We even had a few missionaries, so we are doubly evil on PC charts.
On the other hand, I've heard of other families in Texas/New Mexico and Florida that have been around as long as my family that probably were Jewish at some point (and they may still be, according to your viewpoints on bloodlines and religion).
It's very hard to determine, because a lot of records of that era were lost, or people took new names. As an example, although slightly opposite of the OP, Lute Olson, the Arizona men's basketball coach, his family name was originally "Pugerud" and it changed over the generations for whatever reason (there's an article linked at
Random Genealogy that details it).
Ironicly enough, at least in the case of many Spanish descendants, church records would have been one of primary ways to varify one's genealogy/family history, as they could be quite extensive.