Some people just have this "wall" about them... like they really don't want to be "read", don't want people to know who they truly are. Sad condition to be in..
I have noticed this too. I think that they are people with very little weakness in their personality. They are confident and comfortable with themselves.
There is a study showing that the people that are easy for you to read are people that 1 are vary open to all people or 2 you have know for some time and know who they are.
It could also be that the reading people thing is not one of the strong parts in your personality.
I've been told I'm hard to read.... probably because I'm pretty stoic when people talk about stuff to me.