Aren't those stats counting Hispanics as white? Admittedly, many Hispanics are white. Most of the Cubans in Florida, for example, or Hispanics whose lineage in America goes back for many decades to landowners in the southwestern states we annexed. But their population really doesn't appear to be growing. I would guess they have about the same low birth rate as other whites, and aren't usually the ones sneaking into the country illegally.
Using your definition, a substantial majority of the kids in the Los Angeles public schools would be classed as white. But that's surely not the case. Most would seem to be Mestizos.