The ACPI standard. It says that in S3 some power is retained to preserve ram, and to use S4 if you want to retain system state without power. If the system does not wake back up out of S3 ( to the OS ) to transition back to S4, then the system is technically not following the standard since it was told to use S3, and is instead using S4, while pretending it is still in S3, thus it calls the S3 wakeup vector instead of following the S4 resume path.

Hopefully lieing to OSPM in this way won't cause much trouble. Where I would expect to see some is for devices that are able to wake the system from S3 but not S4. They won't be able to wake the system and OSPM will have no idea why.