My friend get's roughly 2 or 3 less than me when we win the same game.
I am pretty sure that bonus LP means that the system wants you to rank up faster, so that you can play games with those of a similar skill level, instead of sitting at the same rank. I am Silver, and getting 25+ LP a game, I know I can get to gold. My friend, in low gold, isn't getting much less, which means he might be worth high gold or more.
Since we can be about the same skill level, albeit, he's better than me, he knows what skill I should be in. So it makes sense that I should be brought to the same rank with him, while he goes on a bit further.
What I'm trying to say is, your friend is probably better than you.