To answer your question, Phil, gloss whites (and yellows, oranges, etc.) have even less hide than flat whites. In general, a flat paint will have a higher hide value than the same color in gloss. In white, this is caused by use of a different white pigment, a higher pigment load, and the use of "flatting agents"—pigments that increase hide as well as reduce gloss. In the case of other colors, the primary cause of higher hide is the use of a higher pigment load and flatting agents.
There is a limit to the amount of pigment that can be put into a gloss paint before the pigment load affects the gloss of the cured film.