I am playing with SmartGo and I don’t understand one situation.

I am playing as Black.

$$Bcm82
$$ +---------+
$$ |O.O.OX.X.|
$$ |OOOOOXXXX|
$$ |.OXXOXXO.|
$$ |OOOXXXXXX|
$$ |COOOOOXXX|
$$ |2XO.OOOX.|
$$ |XXXOOXOXO|
$$ |nXOOOXXXO|
$$ |1XXO.XyXO|
$$ +---------+

Why am I allowed to play at G1 (marked y), but not at A2 (n)? I am surrounded by my own stones in both cases.