My guess is that our Uruuno might actually be from the Yuri-verse while the one other Uruuno actually is from regular Earth.
The two switched placed due to a lab experiment; and while our Uruuno lost her memories of her old-old life, the other Uruuno retained them (or lost them too and regained them earlier) and wanted to "switch back".
It would explain why their personalities are so different too. The other Uruuno being quiet and calculating was because she didn't feel an attachment to the Yuri-verse, because to her it isn't her actual home.