Hi Jeff, there's a number of reasons:
1. The paper is the wrong colour (but this is also true for some of the reprints).
2. The printing's not very clear (and again this is true for some of the later reprints).
3. The white line above the value 15 is the wrong shape.
4. The little flag shape to the top left of the first letter of Russkaya is the wrong shape.
5. The last letter of Russkaya is the wrong shape.
These stamps have a very murky past but the history of the period is fascinating. The leader of the Western Army was fairly typical of the colourful characters who emerged as leaders of factions in the Russian Civil War:
http://en.wikipedia.org/wiki/Pavel_Bermondt-Avalov