So that story is a take on two different types of isekai. One character was reincarnated into an already existing person—possessing and stealing that person’s life, leaving the original soul in a kind of limbo. That’s why she still has the mannerisms of her old world, being a modern person reincarnated. Meanwhile, another soul was reincarnated into her own body, simply remembering her previous life. She didn’t possess someone else’s body, and since her experiences are real, she still has her own personality, just with the added memories and experiences of her past life.