When does SQL Server follow the Minimum Password Age policy?

According to this SQL Server 2008 whitepaper, SQL Server should only pay attention to the Minimum Password Age policy when the Check_Expiration (Enforce Password Expiration) option is turned on. Page 7 in the whitepaper specifically says "CHECK_EXPIRATION uses the minimum and maximum password age part of the Windows Server 2003 policy, and CHECK_POLICY uses the other policy settings". However, that's not what I'm finding. It looks like you only need Check_Policy (Enforce Password Policy) turned on.

For example, I have a SQL Login (using SQL Server authentication) with Enforce Password Policy turned on, but Enforce Password Expiration turned OFF. My domain's password policy has Minimum Password Age set to 1. I can use sp_password to change the password once, but when I try to change it a second time, I get this error:

Server: Msg 15114, Level 16, State 1, Line 1 Password validation failed. The password for the user is too recent to change.

Is the documentation in this whitepaper simply wrong? Why is this happening when Check_Expiration is off? I also found this site which also says that Minimum Password Age should only be enforced when Check_Expiration is on. I think this is true for MAXIMUM Password Age, but I'm finding that Minimum Password Age is still getting enforced when Check_Expiration is off.

Is there something funky about my configuration? Should Minimum Password Age really be enforced when Check_Expiration is off (but Check_Policy is on)?