The description of resources from the different source databases in LUBsearch can differ (i.e. the description of material type, title, author etc.). To be sure that you are getting all e-books when searching please limit your result list on both Material Types ->Book and Material Types -> eBook.
If you limit your search result to Books you will ge both printed books and e-books in the result list and if you limit your search result to eBooks you will only get e-books in the result list.