[Wowfunhappy]

I know nothing about this, but I imagine two people at the same latitude and longitude could be at different altitudes.

A trivial example would be people on different floors of a skyscraper—although I suppose gps works poorly indoors anyway. Still, even outdoors there are peaks and crevices, and on a steep slope a very trivial change in lat/lon could lead to a major change in altitude.