[Koha] Question on Z39.50 cross-format queries

[Manos Petridis]

First, allow me to present the context of my questions and suggestions: I've recently begun to use coha, trying to document and organise my personal library and perhaps the one of my brother, once I'm done with mine. I don't know much about library science, so I rely on Z39.50 look-ups to fill-in the blanks, literally.
 
Question:
I have selected USMARC/MARC21 as my installation's native format. This serves nicely when I perform Z39.50 queries against many USMARC/MARC21 sources. On the other hand, when I perform Z39.50 queries against UNIMARC sources, I don't get properly formatted results even though I have set-up those sources (origins) as UNIMARC in koha. 

I understand that both standards encode the same more-or-less information, differently. 
Therefore - if I understand correctly - koha would have to request from an UNIMARC-only-capable source that they return UNIMARC-formatted results, and koha would need to map them - if and as possible - to the native format of my installation, i.e. USMARC/MARC21. 

Apparently this doesn't happen. Is it normal?
TIA
Manos Petridis