It doesn't. It increases when the market price increases, it decreases when the market price falls.

The point of the price cap isn't to be some sort of subsidy of consumer energy costs - though the government did some separate stuff along those lines with the energy price guarantee (which capped the typical household's energy bill at £2,500 for the period it was relevant) or the support scheme in winter 2022 when the taxpayer paid everyone £400. The price cap is now below the energy price guarantee so the subsidies are no longer relevant.

The price cap is just a way of giving people who chose to be on variable tariffs a little bit of predictability of what they'll be paying for energy three months ahead when the market prices are moving around.