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When the username starts in lowercase it's stored with that lowercase in the “_sites” map. A site requested via data_repository() is stored with an uppercase starting username.

>>> import pywikibot
>>> s = pywikibot.Site('test', 'wikidata')
>>> pywikibot._sites
{'wikidata:test:xZise': Site("test", "wikidata")}
>>> type(s)
<class 'pywikibot.site.APISite'>
>>> d = s.data_repository()
>>> type(d)
<class 'pywikibot.site.DataSite'>
>>> pywikibot._sites
{'wikidata:test:xZise': Site("test", "wikidata"), 'wikidata:test:XZise': DataSite("test", "wikidata")}

This doesn't make a difference usually, because it requests via the “original” lowercased name:

>>> import pywikibot
>>> pywikibot.Site('en', 'wikipedia')
Site("en", "wikipedia")
>>> pywikibot._sites
{'wikipedia:en:xZise': Site("en", "wikipedia")}
>>> pywikibot.Site('en', 'wikipedia')
Site("en", "wikipedia")
>>> pywikibot._sites
{'wikipedia:en:xZise': Site("en", "wikipedia")}