The rules don't actually mention anything like that, so I'd assume that they actually have their reach of 12 hexes.
On pg. 96 of Interstellar Operations under the Improved Standard Missile Launchers heading, it talks about how the Clans got the 12 hex range:
"Going a different direction, however, were the improved SRMs
that appeared a full year before the new LRMs. Utilizing more
powerful launch mechanisms, the early Clan weapons-makers
increased the SRMs’ effective range by one-third, even though
launchers themselves remained about the same weight and size as
their original counterparts."
Since they're talking about the launchers themselves, I'm thinking it's on the launcher side of things and not the ammo side.