The thing you need to understand is the the Messages app is not quite the same beast on the iPad as the iPhone. Because the iPhone is a cell phone, it can use SMS and MMS messaging as well as Apples own iMessage service. When iMessage is available, the iPhone uses it. If it is not available (and you have it enabled) it will use SMS. The iPad can not. It is limited to Apple's iMessage, which requires the internet.
What this means is that your iPad can only send and receive to/from other iMessage users who have registered their iCloud account with iMessages (in Settings > Messages). The iPhone can send/receive to both iMessage and SMS.
If both the iPhone and iPad are set up with the same iCloud account in Settings > Messages, it is possible to receive iMessages sent to the iPhone's number on the iPad. And that is where your confusion it coming from, because it only works if the iPhone is receiving the message from iMessages. If the person sent the message to the iPhone using SMS, either because they are not on iOS or did not have internet available when sending from their iPhone, then the iPad will not get that message.
Why? Because it never touched Apple's servers. It went directly to the iPhone via the phone network.
So it goes like this.
- iPhone with internet connection sends to phone number -> iPad and iPhone get (because it's using iMessages)
- iPhone without internet sends to phone number -> iPhone gets. Because the message was sent using SMS, directly to the iPhone.
- Other phone sends to phone number -> iPhone gets. Because iMessages is only available for iOS or OSX.
The other thing to hold in mind is that just because the other person has an iPhone (or iPad) does not mean they setup iMessages in Settings > Messages; in which case only SMS will work, and only between phones.
I hope that helps.
