I have always been quite shy/quiet/reserved. However, I have improved a lot. At work I am a little reserved and prefer to listen more than I talk but I still engage with lots of conversations and have good relationships with my colleagues (some I would even class as friends). I can speak up in meetings, ask for help, etc as and when needed.
Despite all this I still get the "you're so quiet" comments that make my heart sink as they prove all my attempts at being more outgoing aren't enough.
A senior at work has mentioned it to me several times. Once when I actively engaged in a conversation with my colleague and he came over and changed the subject and then remarked that I'm very quiet (despite happily chatting away as he walked over), and the next time I was sitting alone in the office when he walked over and said "you're so quiet"? He once also said I'm so quiet he forgets I even work here. I smile and make small talk with him so it's not like I sit there completely silent when he walks in.
I have several colleagues who are also quiet and he never, ever says it to them. It's a pattern I have seen before at university and school where my quietness will be commented on but not others'. I don't understand why.