I'm not asking that all the level 10 weapons be the same, never, its just as Raisu says, technology advances.
All Houses must have a large defense budget in order to upgrade or develop new weapons. Thinking logically, there would be advancements. Factions must have a decent sum of money from their plundering, or other activities. Or theft of technological blueprints...
The level 10 weapons could just be upgraded level 9's, or 'degraded' CODENAMES, since mass producing the current ones would be pretty expensive... So toning them down would make sense to me.