Can call numbers differ between libraries?
Normally, when already available, Library of Congress classifications are usually found after the title page, and should thus be constant across different libraries. For publishers. 

In any case, this should be a fairly easy hypothesis to test (at least to a certain degree). Most university library catalogues are on-line and freely available. You could sample a cross-section of titles from different subjects and see what happens: my suspicion after a few cursory checks of my own is that call numbers do vary somewhat, but primarily at the "secondary level" (in identifying the author, rather than the subject).