I think its a boo-boo in part of HTC.
My guess is that they had intended to allow people to change each instrumental guages of the planes respectively, in the same manner that sound files can be designated for each plane.
Otherwise there isn't any reason every plane textures include guages, when they are all the same in every plane.
Probably when the guage BMP is saved in the texsrc folder, during build process of the map it gets applied to every texture file which has the same name.
I think what we need is a way to save textures as a group. For instance, the sound folder groups the files by directory. Maybe HTC can help us by allowing the BMPs to be saved in groups?