One of my other hobbies is genealogy, and I've come across a question that crosses well into name-geekery: in 19th-century German-language Lutheran records from western Hungary, I keep encountering women who are alternatingly named Theresia and Rosina. For example, my spouse has an ancestor who was baptized Theresia in 1835, but lived her life as Rosina; she was buried in 1889 as Róza (because that church register was in Hungarian). Researching my father's ancestors today, I came across another example, where the equivalence cannot be questioned, because the birth and burial were a grand total of four days apart: Theresia, the daughter of Joseph Kirnbauer and Barbara Neubauer, was born and baptised on Sep. 26, 1838, and Rosina, the daughter of Joseph and Barbara Kirnbauer, died on Sep. 29, 1838 at the age of three days.

What is it about these two names? What am I missing?