I find it strange too. My grandmother moved to England from Italy over 60 years ago but she sounds like she arrived a couple of weeks ago. I think it's a combination of wanting to maintain her Italian identity and talking daily/weekly to other people in Italian. It must take effort, even if subconsciously (e.g. wanting to fit in, thinking it affects the way people perceive you, not being understood etc), because it clearly doesn't happen automatically.