Hello Liam,
Thanks for using Apple Support Communities.
If your usage and standby time are exactly the same, then it might be that you don't have auto lock enabled, thus keeping the iPhone awake. Check your auto lock setting as shown below to ensure it is configured correctly.
iPhone locks automatically if you don’t touch the screen for a minute or so. To adjust the timing, go to Settings > General > Auto-Lock.
Sleep/Wake button - iPhone
The device is awake if you’re on a call, using apps, listening to music, browsing the web, or sending and receiving email, or if an app refreshes in the background. View Standby to see the amount of time the device has been turned on since its last full charge, including the time the device has been asleep.
iOS: About standby and usage time - Apple Support
Cheers,
Alex H.