The Focus Damage Gem and Heavy Gem

The weapon is Snarling Spear
(1) heavy attack (no gem, no gear)                                                                                  damage: 419
(2) heavy attack + focus hit (no gem, no gear)                                                              damage: 1005
(3) heavy attack + focus hit + Yammo's Noggin-Padder (no gem)                             damage: 1307
(4) heavy attack + focus hit + Yammo's Noggin-Padder + Heavy Gem γ                  damage: 1427
(5) heavy attack + focus hit + Yammo's Noggin-Padder + Focus Damage Gem γ  damage: 1463

If I'm not wrong:
for (4): 1005 x 0.12 + 1005 x 1.3 = 1427
for (5): 1005 x 1.3 x 1.12 = 1463

So is that working as intended? Why the  Heavy Gem γ can't benefit from Yammo's Noggin-Padder？