That mist be the format that usse Multi-LZ Compression formatted parts, I know Dragon Quest 9 had this compression method including the section of the anti-piracy area of the code, which is makes it harder to remove the code, but it seems the arc files are a new form of compression used in the arsinal of Nintendo Compression Dev Kits, more so that I think there is one reason to these LZ based arc files,
1, Copy Protection,
2, Impossible to decompress and even decode,
3, Anti-Piracy and Anti-Modification measure,
Something Nintendo is using to battle against the Rom Hackers and Piracy mongers.
And I am sure there will be more unknown Compression Archive formats that Nintendo will come up with to make sure we accept the stuff they give to us, well it seems that is the new plan now.