iPad has more storage Available than it's Capacity...?

I went to order a new iPhone today and thought I'd check what size my current iPhone 12 Pro Max and iPad Pro 12.9 Wifi + Cellular (2nd Generation) are and how much of the storage space I'm using to determine what size capacity iPhone to buy.

On the devices I went to ''Settings', 'General', 'About' and scrolled down about midway. The iPad states:

"Capacity: 256 GB

Available: 315.86 GB"

Now I'm no IT wizard, but it seems impossible to me to have more storage still available for my use than the sum of my devices total storage!

If instead I go to 'Settings', 'General', 'iPad Storage', it states:

"iPad Zero KB of 256 GB Used".

The bar graph visually shows it to be about half full though - which to me indicates around 128 GB or so used.

Similarly, the results on my iPhone seem to be far less used than I would expect. 'About' reads:

"Capacity 512 GB

Available 439.1 GB'

I'm fairly sure that last time I moved up from a 256 GB iPhone 11 to a 512 GB iPhone 12 due to insufficient storage space so I'm not sure where all the data has gone...

'iPhone Storage' reads:

"72.8 GB of 512 GB Used"

with the bar graph visually looking to be about 1/3rd of the way across - which to me would indicate approximately 170 GB or so used.

Both devices have Software Version 15.0.2 installed.

Any ideas on this?