Adding a new hard drive to RAID1 software RAID fails

I've got a Mac Pro 1.1 running macOS Server 10.6.8 that I want to use as a RAID-Server. I use Disk Utility to run a RAID1 set with two 500GB hard drives.

Now adding a third 500GB hard drive to the existing RAID-Set with diskutil appleraid add member fails by showing me the following error message:

Error: -9911: The target disk appears to be too small to add to this RAID set

But when I delete the RAID-Set and set it up with all three hard drives together, the RAID is being created without any problem.

I belive that it could have something to do with the fact, that the third hard drive is a different model (different company), and the other two are the exact same. But if that would be the case, then I could not replace a hard drive in a RAID-Set, even if it would be from the same company and the same model line – since even those can change over time – except if I buy the exact same one – right?

I want to be sure that Apple RAID is capable of adding a new drive to an existing RAID if I have to swap a damaged one. Is anyone able to help me with this problem?

Thanks in advance!