Just to point one thing out: This thread doesn't consider any shenanigans or fraudulent operations to skew the outcome or the classification.
My key example: What happens when Duolingo assigns an Android user to a league filled with website users? I have removed all league nicknames but my own. The below result is on Tuesday at noon (UTC). The top Android user scores 5816 XP. Only one website user surpassses 10% of that total.

The DuoMe raw data of the first user shows the usual XP farming: a sequence of 60XP legendary repeats or tests, during a triple XP interval. That makes 6 times 180 XP + once 60 XP after the triple ran out. Several hours later a few lessons or repeats were added to complete the daily quests. Now a series of 120 XP events (again triple XP) follow. After triple time ran out, the user completed a few useful ordinary lessons at 20 XP. Note that this user has no SUPER subscription!
What happens when DL assigns one website user to a league with Android app users is also obvious: the user will be demoted, no matter his effort.



L25 (Duo) / A2 (6+y, McGraw-Hill) - Learning (Busuu):