Re: 104% of what?
The max listed is not to be used as a SAFE RANGE.
The max listed load is where the cartridge used and components used reached the maximum SAAMI working pressure for that cartridge in that test barrel.
Some individual rifles may reach maximum pressure before the maximum listed load is reached. Other rifles may allow the loader to exceed the maximum listed load safely.
Also, fired cases and different brands of cases may have more or less capacity. It all makes a difference.
The 104% charge of case capacity means the particular cases used with the bullet used at the overall length used had a certain measured capacity and the powder charge exceeded that capacity. The charge would be compressed.
I have seen where a listed over capacity load wouldn't even fit in a case without a bullet. Many of these loads if used need to be charged with a dropper tube while dumping the powder to create a spin in order for the powder charge to fit.