No it's actually the one reason why I like the iPhone and specifically push notifications so much. It is delivering to their phones immediately, assuming they have a data connection active, but the way the app works on iOS means that you have to physically open the application for it to register a message receipt and thus give the sender a second tick.
For example, let's assume you send me a message. You'll get a tick and as soon as the message comes through to me I'll get a push notification with the message on my screen. At this point though, you will still only have 1 tick. It's only when I actually click on the notification or open whatsapp itself that you will get the second tick. So technically I can receive and read a message via push notifications without you even knowing.
Hope that makes sense?