If it weighs the same as your H buffer it isn't the 9mm version
IIRC the 9mm buffer weighs substantially more
But are you giving it the "one in each hand" weight comparison? When it comes to a n ounce here or there, its difficult to be precise.
There is a very informative buffer discussion here somewhere, I will see if I can find it to shed some light..........................
This one starts out on other buffer types but gets around to all of them : 14 pages
This one has a couple of pictures
and lastly, if you can weigh yours with some degree of accuracy this may help....(credit to Slash)