How do certain apps show new notifications when internet access is restricted to them?

I have disabled WhatsApp's access to Internet by disabling both WLAN-usage and mobile network usage. I thought this should block WhatsApp from accessing the internet. But miraculously WhatsApp gives me a message with ominous content: "You might have new messages" When I open WA it does not show any messages. When I then give it back it's access rights wonder what, there are messages I received the same day! So to me it looks as if this app has found its way to sneak around my network restrictions. So the following questions arise: * Is my assumption about the effectiveness of restricting the "data usage" for apps wrong? * Does WhatsApp hack around the standard restrictions?