win10 SW raid? losing drive space?

Running an older dell desktop (w10 home). i slapped a ASM1064 pcie sata controller. No issues seeing. 6 x 3tb drives on that card (can format, use individual disks etc). when i go to create a sw raid using all 6 drives the total storage remaining is only 10,8 tb using parity option. why so low when all other systems give me about 14 and change tb? if i use simple it gives me 16.3 tb i expect to lose 1 disks worth of space but this is almost 2 full disks . is this the only option or am i setting something wrong up?