Really Cheap (And Sketchy) Calling Cards

You know what I'm talking about. Those $5/76312345235 minute calling cards 'on sale' at convenience stores and other sketchy retailers. How are they so cheap, and generally how reliable are they? I've bought a few, and two were fairly close to what was promised, while another didn't work. Did I get lucky, or unlucky? Are these stolen or something? I currently pay like $0.30 per minute for long distance and these cards promise something like $0.07 a minute, so they're tempting, but I'm curious what's really going on...