Loki Luv, MD° (no login) Posted Apr 6, 2012 12:02 AM
Sometimes I think the people that run 'em deliberately break up the sets if they notice that the multiple volumes belong together.
Even fiction... ...I can't recall all the times I've picked up some peculiarly interesting title only to find that its Volume Whatever of The Whosamawhatsit Saga and of course it's the only one to be found.
Although one can always look for any others on Amazon.com if you want 'em bad enough. No guarantees, of course, but at least there's the option.