Part of the capacity difference is due to different operating procedures. U.S. carriers park aircraft on the flight deck between air ops, and sometimes even during. The Royal Navy has always stored their aircraft on the hanger deck. That has historically resulted in smaller air wings.
The USN carrier fleet came of age fighting the IJN. We needed every damn airplane we could muster ... old habits die hard. This one shouldn't die at all.
If your operating in the North Sea during winter it makes a lot of sense to keep all your aircraft below deck.