Spanning mode with two different type and size of hard drives

WD Legacy Products My Book Live Duo
1

Hi Everyone!
I configured My Book Live Duo Personal Cloud Storage with two different HDD in spanning mode
Two HDD’s are mismatched in size (8TB & 3TB)
Created volume shown as 6TB
What’s wrong?
In documents for spanning mode two size must be concatenated

2

Anybody can help?