Hi Sage,
This is my interpretation of what happens.
The user.lib file stores all the textures that are used in a course and if you use the same APCD file each time you open a course file all textures in that course file are added to the user.lib file. I believe(?) that if you have texture named "rough" in the user.lib file and the new course file also has the same name the existing rough texture in the user.lib will be seen in the new APCD file and not the texture that is downloaded.
I hasten to add that this is my theory and may be pure bulls**t.
If you use the same APCD file for each course, only the textures used on that course will be stored in the course user.lib file.
I hope that a real designer will explain the real reason.
Ian