Good in or Good at?

I was recently engaged in a discussion with a Bulgarian english teacher on Skype. I was telling him that I was good IN pronunciation and he immediately corrected me with "good AT".
I was puzzled by this, so I googled about it.
So I found this BBC english place
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv8.shtml

It was written that there's almost no difference between these 2. But the bulgarian friend insisted that he was right with "good AT".
Althought "Good at sex" is right, but "Good at chemistry/pronunciation/medicine/bed" sounds not right to me.

So anyone? Enlighten me please!!!