My ring fingers are different ring sizes, so it can happen, and it's actually the non dominant one that is bigger. But! It's not like you can't alter a ring to fit. She wants people to be confused. I bet that in her head, randos on the street look at her and think she's engaged.
She leads a sad existance where external validation is all she seeks, so I wouldn't put it past her to derive some pleasure from appearing to be engaged.
It's still a shock to me that she reproduced with a man who not only has no obvious intention to marry her but who has dangled an engagement multiple times and used it for views.