The problem with the (popular) g3 opening is that is was not given a specific code when the Encyclopaedia of Chess Openings was written. Instead it was grouped with the irregular flank openings which are all A00. Most g3 games will transpose to openings with another code, in your game it transposed to C20. Since your first move was g3 the code is A00, but had you switch around your first 2 moves the correct code would have been C20.
365Chess.com will give you the correct codes for openings.