Yes, I believe its a batch or quality thing.
Most G3s have some kind of code or number its just than on some of them its really apparent and visible to the naked eye - if you have a pony where you cant see the number if you rub your nail over the centre of the hoof which doesnt have the "2002/3 Hasbro" or "Made in China" stamp or the hoof magnet you ll find a little dent/scratch in the plastic - I am sure if we were able to see it better there is a number covered over or embedded so deep in the plastic its partially been covered.