unraid works unlike raid 5 where parity is striped between all the drives.
In Un-Raid it looks at the exisiting bit in all the drives to calculate the parity bit.
So with 4 drives +1 parity, it would look at the first bit on all 4 drives and calculate the parity bit and write it. then repeat for bit 2, 3, 4 ...
When you insert a disk to the raid, it fills the drive with 0's as a 0 has no impact on the partity calculation.