[Q] Notifications arrive during sleep

I'm running on stock 4.1.2 and mobile data is always turned off. "Keep Wi-Fi on during sleep" is set to "Only when plugged in" so the phone should only connect to Wi-Fi when it's in use. It seems that the phone periodically connects to Wi-Fi even though it's been idle for extended periods (e.g. an hour) because it's able to deliver notifications (e.g. gmail and hangouts). Other Android devices that I have, including Nexus devices with the same settings don't deliver notifications until they are "awoken".

This might also explain where a lot of battery goes. Does anyone else experience this and does anyone know if other phones/brands/models behave like this?