No, it isn't, or, if you prefer, it only happens on the vast majority of systems, but not necessarily on all of them.
The booting PE has been documented to in some cases to detect disks in a different order from BIOS, and of course another number of factors may be involved, like the exact way the disk is partitioned (before or during the install) and the exact way the OS is installed (via normal setup, third party automated tools, dism apply, etc., etc.).
Since it costs nothing at install times to disconnect the "other" disks it represent a foolproof way to make sure it doesn't happen.
jaclaz