I think it's because Jews are also an ethnic group
"To ensure that we did not overlook religiously unengaged Jews, we asked students for their religion and whether they considered themselves Jewish aside from religion; then, we asked about their parents’ religious identities. Students who considered themselves Jewish in any way were counted in our estimates." -- https://ejewishphilanthropy.com/how-many-jewish-undergraduat...
"Jewishness in the United States is considered an ethnic identity as well as a religious one." -- https://en.wikipedia.org/wiki/American_Jews#Religion