600 is 600, but the amount of effort you put into it can make a tremendous difference in the final true grit. How much pressure you use and how long you do it can give you anything between 600-grit and the stock 4000-grit. It's much easier to get the real 600-grit with a spinner than by hand, and if you don't put the time and effort into it, there's no telling.
Best thing is to decide if you like the reaction you've gotten with the green pad and if so, maintain it by being consistent each time.
SH