Just curious, but why do demographers segregate "hispanic whites" from "non-hispanic whites?" I don't recall anyone segregating non-Italian whites and non-Irish whites from Italian whites and Irish whites.
The problem is that demographers and policy makers would like to consider "Hispanics" as a race like Asians, Blacks, and Whites, but it doesn't work clearly because all Hispanics have in common is language. On Census forms, given a choice, many of them will check off "other", "white", or "native American" because that's accurate for them but it makes the phenomenon of Hispanicness untestable. It has to be checked separately.
In other words, the difference is that there are no Italians who are Native American, Black, or a mix of races like most Hispanics are.