pdf in ibooks sorted in collections, sync with ipad fails

I am using latest version of iBooks on an iMac with Yosemite 10.10.5, iPad 4 and iPhone 6 with latest IOS 8.4.1.

In iBooks on my iMac I habe sorted many pdfs into collections. I want to sync the complete collections to my iPhone and iPad via iTunes.

This is ok at the beginning. When I add an additional pdf into an existing collection on my iMac the sync is not correct. The pdf is then not visible in the collection on my iPhone/iPad but only in the overall pdf view. Since I have almost 100 pdfs this is a nonsense.

I have enabled sync of collections.... on all devices iMac, iPhone, iPad. I am syncing via iTunes via WIFI. I do not want to bring these collections into an icloud.

What am I doing wrong?

IBooks seems not to be a highlight in usability. Editing Categories in iBoolks is a nightmare, works onyl in list mode 1 by 1.

Why Apple seems not be able to provide a reasonable sync?