When trying to a refine search for books only (no ebooks), I noticed nothing was happening; I was still geting all books AND ebooks. Upon further inspection I realized that the authorized values for books was BK and eBooks was BK-E. I realized that Koha was probably simply not being very strict when narrowing results by item type and was returning BK and BK-E because they both contained the string 'BK'. I tested whether this was the case with other item types and didn't have an issue. I then tested by creating a new item type with authorized value EPUB and manually switched some BK-E records to the new item type (both 942$c and 952$y) and that resolved the issue.
That's because - is treated as a space. So the search for BK is ALSO a search for BK E. This has been reported often, but is actually more of a configuration issue. It's recommended to not use spaces or hyphens in your itemtype, collection and location codes.