Assuming user shares have default Allocation Method of Highwater, what you have is normal. You can adjust the warning level for each disk as seen in that 2nd screenshot you posted.
Disk1 has 4TB free, so disk1 is chosen first, and Highwater is set at half the remaining capacity of the disk, 2TB free. So it uses disk1 until it has 2TB free.
Then it re-evaluates Highwater.
Now disk1 has 2TB free, and all other disks also have 2TB free, so disk1 is chosen first, and Highwater is set at half the remaining, 1TB free. It will continue to use disk1 until it has 1TB free, then go on to disk2 until it has 1TB free, and so on.
Other allocation methods are available, but Highwater is default for good reasons. It is a compromise between using all disks (eventually) without constantly switching disks, as would happen with Most Free Allocation.