It's like I don't even know you

The oddest thing happened to me last night. I was talking to a guy I dated for like 3 years and I decided to ask him some questions and see how well he actually did know me. The first thing I asked him was what was my older sisters name. It didn't surprise me much when he didn't know. I had to throw random names out there before he finally guessed the right one. He's met her on numerous occasions. It made me angry. I'm not sure what to think. Did he just ignore me when we were talking? Or is he just incredibly forgetful? It just doesn't seem to make sense to me, how can you date someone for 3 years and not know something so basic. After that question I asked him a whole bunch more, some of which he got right, some of which he didn't. Then he asked me some questions - and to my surprise I didn't even know the answers to half of them. However I did know the family ones!! But he asked me about his favorite book. I honestly didn't know he had one. He's never told me he's in the middle of a good book, or that he finished one. The only thing I've ever seen him read is a car magazine!
I guess it just astonishes me that I invested so much time into a relationship with a guy who I actually don't know much about. It makes sense when I think about it. Whenever he and I go out to dinner or even just drive somewhere we never really have much to talk about. I guess that's just because we don't know anything about each other.
I wonder how it happened because at the beginning of our relationship we always had so much to talk about. Stories to tell each other and everything. I suppose after a while we just stopped talking and started to drift more than we really noticed. But I think he and I have turned into two very different people than we were when we started dating and that we don't even know each other anymore. It doesn't even bother me. You'd think it would, that I'd be upset that this person I care so much about doesn't know me. But it doesn't, it doesn't phase me.