Material.setBumpMap() should be renamed Material.setNormalMap(),
as the map used has an RGB value indicating XYZ normal direction.
The map currently used by setBumpMap is a normal map as described in this link:
http://wiki.blender.org/index.php/Doc:2.4/Manual/Textures/Influence/Material/Bump_and_Normal
These maps definitely have distinct x, y and z values (normal) and changing those components individually has an impact on the visual result.

In animation production, we use 3 kinds of maps:
[*] Bump Map : Greyscale value indicating bump
[*] Normal Map : RGB value indicating XYZ normal direction
[*] Displacement Map : actually moves the surface of the geometry (expensive)