Let's talk about what happens to those old books, the back list. Made way back in the day when computers weren't so common. Yeah, those books.
First, to create computer files, a small press might have Kinko's chop the spine off a paperback edition. Then the loose pages get scanned.
Second, someone goes through the file and fixes as many really egregious errors as possible. Things they know the scanner screws up. Like words the scanner didn't understand.
A lot of what slips past then gets fixed during proofreading.
Back in 1989, when I worked for a research consortium, the scanner had two languages and whenever it got confused, it liked Greek best. So I'd have to go through and fix every triangle starting a sentence and make it into a capital A. (But I'd keep the Greek inside formulas.) And so on.
After the book is scanned, then it goes to a proofreader. She's going to compare the files to the original manuscript.
I know because I've done this often. I watch for letters run together, or dropped off, for altered words and punctuation...
You might not think proofreading is important. It is very important. Say you loved Dune. Every ebook version of Dune will want proofreading. Because that book was definitely not written on a computer.
Every time ANY book has to be converted into a file, it has got to be proofread. Remember, spellcheckers don't notice altered words -- how could they? -- as long as it's a real word.
This is also why even though you might have books in print, those titles don't immediately become ebooks. Ebook conversion isn't a magic process. It takes time. [Read Toby's link at the top for details.]
Charlie sums it up:
"There is no topic in the publishing industry this decade that is the source of as many misconceptions, superstitions, lies, plausible untruths, and idiocies as ebooks."
Postscript: I don't understand readers who think "it's not physical" means it must be cheaper. They pay for cellphones and cable. They can look at their devices that provide these services, but they pay for those services as if the Thing-in-hand equals the Service.