iMessage Question/Issue

Hi All,

I did search on my issue, but found the number of (irrelevant) results pretty daunting to navigate through.  I'm sure I have a setting incorrect or just misunderstand how iMessage works, etc., but here's the situation:

On two occasions recently this has happened...I send a text from my iPhone to a friend, who is also an iPhone user.  His response shows up in Messages on my iPad, which is sitting at home.  I don't receive the response on my iPhone.

What am I doing wrong?  And I guess it's possible that it's not me, as I've only noticed it with two friends and they're not the only ones that use iPhones.

If it matters, I have 6.0 on my iPhone, and 6.0.1 on the iPad.

Thanks!