Can call numbers differ between libraries? Suppose you have two libraries that use the same classification system and have not merged their respective databases. Could it be possible that the exact same book could be assigned different call numbers if taken from one library and added to the other?

I am asking this question because I am drawing up a home-library database application, and the client would like the ability to assign call numbers (even multiple ones). I have not been able to find an answer online, and I hope that I do not have to travel to the next state over to find a library not connected to the ones around I live.

I imagine the book would most likely be assigned to the same top-tier category and most likely the same second subcategory in the respective system, but, for example, the Library of Congress system incorporates the author's name and a quasi-decimal number which fine-tunes the book's location. And it seems without any external reference the assignment of this part of the call number is made on the fly for any particular library.

So, I have a couple questions. First, the title's sake. Second, is the way I'm thinking about this correct? And if I'm wrong, why?
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).