It can really be anywhere. Intel/AMD probably have done due diligence in ensuring that the SoC itself sleeps correctly, but once system integrators start gluein

It can really be anywhere. Intel/AMD probably have done due diligence in ensuring that the SoC itself sleeps correctly, but once system integrators start glueing on external hardware like radios, external graphics, floppy drives ( :P ), things get messy fast. For example, a wireless driver which doesn't correctly power down the radios when the system sleeps will end up firing interrupts at the SoC and dragging it back from S3 constantly. Or maybe the SoC does actually power gate correctly but the external graphics doesn't and so you end up burning a non-negligible amount of power over there for no good reason. And so on and on and on. Integrators here are really on the hook for validating their configuration and chasing down the appropriate vendor to fix it (be it Intel/AMD or, I dunno, Qualcomm).