How do LP gains/losses work?

Before I begin, sorry if this is in the wrong board.

Anyway, I thought that LP gains/losses were based on your MMR and the MMR of the team youre against/with. For example, if you had a lower MMR than the game average, you'd gain more LP. If you had higher MMR and lost, you'd lose more LP. Apparently this is not the case, as I duoed with my friend who is Gold III (I'm Silver I). After we won, my friend gained 20LP and I gained 17. How does the system decide LP gains?

Thanks. [slayer-jinx-wink]