Diff is forward differencing. In R, it's a difference between i and i+1, but they first organize the data into an array by year, so it really ends up being a difference between month i and month i+12, as I mentioned above. I coded this up myself and got an extremely similar result. (I used the Mauna Loa data which I had on hand, which is slightly different from the global data, so it's not exactly the same.) It does have the right magnitude.