How well can you ever know someone?

Earlier today I stumbled upon a blog belonging to my girlfriend of 2 years, and I was completely flabbergasted by it. It wasn't that she was making comments about me or that I found out she had been engaging in some less than salubrious activities, it just seemed like a completely different person writing than the one I know.

My girlfriend is not the most articulate person in the world, and often has a lot of trouble expressing herself in meaningful ways; sometimes I have hour long conversations with her about things her dog has done today, because she isn't confident in expressing her thoughts. However the blog entries I found, as well as a few emails to various fan websites, showed a depth of intelligence and emotion I had not seen before.

This led me to question exactly why she has never shown me this side of herself, and left me wondering how well I really know her at all.

NB. I found the blog because I was trying to find the meaning of an online handle she uses (it is a Japanese name I think), and I know it is her because posts she made correspond to events in real life.