Because the authority record subfields 152$b (UNIMARC) 942$a (MARC21) are used for the authority type. And they won't disappear if hidden when an authority record is saved. (tested on the 942$a and few colleagues remember it's the same for 152$b) It seems that exemptions for these fields are hard coded. These false positives create noise and might lead to errors where users would think that they need to make them visible. So they could be filtered out unless there is a reason to keep them, right? == Going further == Maybe it's for another bug if these ↓ cases are not as clear as the other ones. But if it's clear then better one bug than two :) 001 is the RECORD IDENTIFIER and 100$a (UNIMARC: General Processing Data) is recreated if missing. So same question for these.
Just tested to show and hide and save 100$a and 001. There is no data loss.