It stuck him that he knew more about people than he thought.
Albert thinks that he knows Ben well. Ben feels that Albert doesn't know him well and I probably know him better. Ben doesn't feel close to Albert although Albert feels that they're good friends.

Carmen thinks that she is friends with people but doesn't realise that they're merely casual and convenient acquaintances, together simply because they don't have other alternatives.

Dulcinia and Edward both shared with him thoughts that they didn't share with others. But why? Is there something wrong with him? Probably because he doesn't judge others, at least not that harshly.
And it occurred to him too, by knowing so much about the people around him, that he actually knew very little about them.