Why system report and diskutil report 28GB instead of 32GB for the SSD part of my iMac 1TB Fusion drive?

Hi,

I bought a 27" iMac Retina 5K (iMac18,3) with the 1TB Fusion drive. On the Apple store site it's written:

"The 1TB Fusion Drive pairs a 1TB hard drive with 32GB of fast SSD — enough to store important macOS files and applications to ensure fast start-up, near instant wake from sleep and quick application launching, with room left over for your most frequently used files and apps."

Source: Customise iMac - Apple (CA) (then click on the link "How much storage is right for you?")

If I check in System Report it clearly show 28 GB:

And diskutil show the same thing:

I also bring the iMac at the Genius Bar and asked them about this; they told that their diagnostic tools show an SSD of 32GB. They didn't want to check in diskutil or System Report. Strange...

For me 32GB it's already really small. If you reduce it by 12.5% to 28GB it doesn't help. This iMac is the family computer and we have four accounts running on it. We suffer regular slowdowns and beachballs.

But my main problem is that Apple didn't gives me what I bought, a Fusion drive with an SSD of 32 GB. It's false advertising, No?

What should I do?

Etienne