Unity300, R6 10+2, hotspare is not needed?

Hi

I am using midrange sizer to make a Unity300 configuration.

I chose 3.5 DPE, 6TB NL-SAS DRV R6 10+2 with system data, just fill out the 12 slot DPE.

Interestingly, it told me no hotspare drive is needed!

As far as I know every 31 drives in a traditional pool come with 1 hotspare per best practice guide.

So is this a bug of sizer or an exceptional hotspare-free configuration?

Thank you.

Michael