Let's say you're right and I'm not saying you're not. It could be that the iPad is using more data due to streaming in a higher resolution due to it's bigger size. I'm not sure about this so if i'm wrong be sure to point out but logically speaking, it makes sense that the iPhone may stream a 480P resolution video while the iPad may stream 720P. Since I do not own an iPhone, I'm not sure whether it is capable of streaming at 720P or not so once again, if my logic is wrong be sure to point out.
No the logic is all there. I'm figuring streaming on an app takes less data usage. I'm still annoyed that AT&T took away my unlimited...