The price cap only applies to 'variable' tariffs, not 'fixed' tariffs. (Every energy supplier must have a variable tariff - here at Octopus, we have our 'Flexible' tariff.)
If you sign up for a fixed tariff, we've got to procure the energy you're going to use ahead of time. If a fixed tariff is more expensive than the price cap, it's because that fixed tariff reflects the current price of wholesale energy.
It's important to note that if fixed prices remain higher than the price cap, then the price cap will likely rise next time time there is a review (every three months) to reflect those high wholesale prices too.
If you’d like to confirm which tariff you're on, and details like your current prices and contract dates, you can see them on your online account (scroll a little way down and look at the details below your property address).