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.