Glow's calculations

I'm curious to know how does Glow come up with its calculations under Genius in the Cycle tab, like is it based on averages of a user's data or what. I've been noticing for a long time that while my luteal phase actually averages around 11 days Glow continues to say my average luteal phase is 13 days, and this always throws my predicted next period off by 2-3 days.