I read a lot from the public library, and I particularly read ebooks. One thing that occurs semi-frequently is a long series of books will have one or two (say out of 10 or 20) that will be in different formats. Say 1-3 will be Kindle, OverDrive, and EPUB. Then 4 will be OverDrive and EPUB only. Then back to the original three. Then 9 will be in Kindle but not EPUB.
When I notice it, I look it up to see if it’s actually available in the missing format, and most of the time it is. So why does this happen?
I’m academic, not public, so this answer may be worth exactly what you’re paying me for it.
I suspect it’s tied into what formats a specific vendor made available at the time of purchase. For example, if I’m buying an ebook, in our purchasing system through YBP, I might see versions from:
ebsco books (who I think bought out ebrary, but the books are still separate somehow)
and one or two more vendors.
Depending on our contracts, we might be able to buy ebrary books, but not ebsco or the other way around. Ebrary might have epub and mobi, but not overdrive or something else.
All this said, hopefully one of our public library folks will be able to speak specifically to issues that could make this happen there.