The Mysteries of Overdrive and E-Books

     I have made the connection with my DC Public Library by using the app "Overdrive." This enables me to put a hold on books and get them down-loaded to my I-Pad when they become available. It seems to work very well, although sometimes the wait for a book seems endless. Recently they have introduced a change that is very illuminating and gives a better idea why the waits are so long.
     Up until recently, you could simply check where you stood in line for a book. It was a bit disconcerting because, for example, I could see that for "Nutshell" by Ian McEwan I was 66th in line...or for "Hillbilly Elegy" I was 236th in line, which makes one think "forget it: cancel the hold." Or go into my account and put a hold on the print copy, which is--after all--more conducive to reading in bed than an I-Pad screen, which is supposed to make sleep more elusive.
     Suddenly they have started to add how many copies of the 
e-book are available, and the results are quite interesting. For one novel that was very well reviewed, there are three copies available. For another novel that was on the NYTimes 100 best for 2016, there only two copies available. For "Nutshell" there are 12 copies. For "Hillbilly Elegy" there are 51 copies. For Ann  Patchett's novel "Commonwealth" there are 42 copies but I am #664 (!!!!) in line. And for "The Association of Small Bombs," where I am 139th in line, there are 11 copies. 
     I am intrigued by the variation in the number of copies. Is it what the publisher allows? Or is the Library's judgement on potential demand?