There is a tendency for lots of people to only talk about themselves, so if your character is such that you don't readily offer up information, and you don't have decades of minor question responses to have built a picture, then it's natural that people have some big gaps in knowing who you are. Newer friends may seem to think they know you, but usually they only know one aspect of who you are, and remain oblivious to the rest of your history, character and dreams.
Many of my work colleagues probably know me better than school mums or neighbours, because the work connection means they ask a broader set of questions. One school mum I'd known for 3 years from nursery onwards, when I mentioned rushing from work. "Oh, I thought you were a stay at home mum." she said. 3 years! She's been to our house, we've been out for drinks, we've chatted most days etc etc etc. Yet she didn't know the most basic info. Not to mention that she won't know my age, where I'm from, my family, financial situation,... anything really.
There are people who truly know me know; DH, DS, and 3 very old friends (2 men I've known 20 years, 1 woman I've known over 30 years). They know a lot, and for whatever they don't know, I don't think anything about me would be surprising to them.