"Apple snail" is a blanket term usually used to refer to all species in the genus Pomacea, the same way "dog" refers to any of the species - german shepherds, beagles, boxers, mastiffs, etc.
"Mystery snail" usually refers to Pomacea Bridgesii - but it's not unusual to see other species mis-labeled this way. However, the ones you'll find at Petco or Petsmart are definitely Pomacea Bridgesii.