Doctor in Scientiarum Socialibus

I have a diploma from the Pontificia Universitas Gregoriana – Doctor in Social Sciences. It reads:

……..“Inter auditores ordinarios Facultatis Scientiarum Socialium fuit ascriptus”…..

and below:
“Nos, eundem (name of graduate) Doctorem in Scientiarum Socialibus” declaramus et renuntiamus……


I can understand “Scientiarum Socialium” all in genitive, because it is “of Social Sciences”

But why “Doctor in Scientiarum Socialibus”? shouldn’t it be ablative, and be “Doctor in Scientiis...

Doctor in Scientiarum Socialibus