OK, I was just wondering if the heavier board might be a heavier construction and you did not know about the difference.
Since both are wood, it may simply be that the full thickness hull of the Isonic is heavier
(as I would expect it to be).
Can you find a scale and test the weight against what the Isonic 111 is supposed to weigh?
Also, are you weighing them with the fins or not.
If your Hypersonic 105 has a carbon fin and the Isonic 111 does not there's a fairly significant weight factor just in the weight of the fin.
I think the amount of sandwich material (a couple of layers of divynicell with a bit of very light glass in between) is going to be nearly the same as the flat bottom of the Isonic. I'm not sure that the overall density/weight of the cured sandwich "skin" is all that much greater than the density weight of the core material.
I have had a number of Hypersonics ( Wood Construction) and they were all very light in weight.
The original Hypersonic 105 (2003 vintage) had a weight spec. of:
Weight Wood 6.9 kg =/- 5%
The 2008 Isonic 111 (W 68) has a weight spec. of: 6.75 Kg.
So, it sounds like you either have a very light Hypersonic 105 or a slightly heavy Isonic 111 (W68).