That took me a second to understand what you were on about.
I think it's just an unlisted bonus of the high-explosive ordnance perk, because currently none of the rockets in Destiny 2 can hold 2 rounds at a time, and usually the "magazine size" they refer to in these types of perks are the number of rounds loaded in said weapon, not your reserves.
EDIT: This seems to be the same across all rockets with HE Ordnance. I did some testing with the Pit Launcher and I can go above the normal 6 rocket cap, but only to 7 rockets. Not sure if this is intentional.
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Ok, i checked it up again to see whats happening without "Heavyweight" Modificator for HC Strikes. The Button has definitly 8 Rockets (eight Rockets) to use when you have the Perk [u]High-Explosive Ordnance[/u] This weapon's projectiles are larger and travel faster. • Increases blast radius • Slightly increases projectile speed [b]• Decreases magazine size[/b] (must be a Text error)