Why can't the account password be same as the iCloud password?

When you install macOS (10.12 Sierra), enter your iCloud credentials and then get prompted to create a local account on the computer, if you try to enter a password that matches that of the iCloud account you will get an error stating the the local account password cannot be the same as the iCloud account password. Why does macOS install not allow you to set matching passwords?

(It is however possible to enter a different password at first and later change it to match the iCloud account, so it appears there is no technical obstacle to using matching passwords.)

Please note: I'm not interested in comments about using the same password being a good practice or not. I'm interested in knowing why the macOS installer enforces different passwords in this specific case.

Absent any explicit statement from Apple, it's as good as it gets: The reason it enforces different passwords is because it's a good security practice.

The fact you are able to change it later shows that they haven't implemented cross-checking in the later stage but it also shows there are no technical reasons why it has to be the way it is.