I'm confused about XP numbers. Yesterday I had several hundred XP; today I have 74; a week or so ago they spiked up to around 2,000 and then fell back down. How are they counted, and why do they change?
They are counted whenever you do lessons or translate in Immersion. The more you learn, the more they increase.
Are you looking at your leader board for weeks? It doesn't display the amount for the last seven days but resets at the same time each week. We just started a new week which is why your xp has gone back to 0. It is the same for months; they are calendar months, not the last 30 days.