There are several types of mandarin oranges grown in China and the rest of the world, differing in size, taste, and color (some are largely green when fully ripe).
All are the species Citrus reticulata. The main types-

1. Mandarin- many cultivars, several cultivars grown in U. S. A. and more around the world.
2. Tangerine- includes Clementine, Ponkan, etc.
3. Satsuma- originated in Japan, and their exported type. Several commercial cultivars.

Dianavan, this explains the differences that you note. Most originated through selection and cross-breeding, some centuries ago.

http://www.hort.purdue.edu/newcrop/morton/mandarin_orange.html