A nine month old's reaction to strangers

Every time we have a new face over at home, The Seniol and I love to play the guessing game as to how LG will react to their presence. Mostly, we both have the same opinion. But there are occasions when he surprises us by his cranky behavior and body language that leaves us to analyze what put him off. We just blame it on one-of-those-people category or just a bad day. There aren't many people in the one-of-those-people category, though. The non-fussy child that he is, we are generally very happy to have someone over - be it the maid servant's grand children or the cook's nieces or friends of The Seniol. He is very happy to be in the company of new people and enjoys playing with them after taking his initial 10 minutes to assess them. He would keep a safe distance look them over up and down a dozen times before inching towards them slowly. I think this is normal behavior for a nine month old who is just trying to distinguish familiar faces from unknown and understanding the difference between family and outsiders.
There is a child who is a little older than LG in the one-of-those-people category. She is a sweet child but I fail to understand what puts him off. They have had enough opportunities to bond and we visit each other's places quite often too, so it's a new face. For a child who coos at strangers on the road to attract attention and smile back at them to pulling the hair of a child much older than him at his first ever meeting, this behavior is beyond my comprehension. He is quick to turn his face on meeting this other kid and doesn't acknowledge her. Certain things are better left unexplored.