Habit , possibly a comfort thing dont read much into it, if you want to stop her using the bottle you can introduce a
sippy cup ,but you know there are no rules on time frame and if you are okay with it, doesnt affect anyone.,when you want to start her off with the sippy cup in the car ,she may need the comfort of the bottle more at bed time.then when you feel right about doing it, just let it get lost somewhere she will miss it for about 2 days...