The cams are different. The 2001 Sno Pro cam is part number 0691-922 while all cams from 2002 to current are 0961-914. The 2000 and 2001 standard 120s also have the "sno pro" cam, but that's not what they were called when in production. It's only been coined "sno pro" since the 2001 sno pro 120 was built.
I've also verified this directly through Arctic Cat. That's also why ISR has it specifically mentioned in the rules that you can swap cams to the Sno Pro cam. The rule wouldn't make sense if all the 120s already had sno pro cams.
Bottom line: If you have a 2000 or 2001 120, don't waste your money on the "sno pro cam" since you already have it. If you have a 2002 or later and racing in stock or improved stock, then you'll see a performance gain.