That can be explained; actually.

Try as it might; you're not going to be in a game with 10 people with identical MMR/ELO. One team is going to be favored over another.

Losing as the statistically disfavored team is going to lose you less LP then if you lost as the favored team. Likewise; winning as the favored team is going to net less LP than winning as the underdog.

Is that true though, or is it based on speculation? I know that LP gain used to be mostly dependent on MMR/what used to be ELO. It was easy to see that when you get matched with mostly plat 2 players while you were gold 1, you would gain a lot of LP so that given enough time, your rank will catch up to your MMR. This was the largest factor during the last seasons. I've never actually seen proof that MMR disparity between the teams was a factor. I think your actual ELO gain however did use to depend on team disparity, so I guess by extension it might play a role, but not necessarily on a game-by-game basis.

Anyway, my point is that any explanations at this point are theorycrafted since Riot has been changing the system so much that any transparency is out the window. That is the real issue.