I took UA872 across the Pacific today (2/21). Before departure, it projects on time departure, 12+ hrs flight time, and 1h15m late arrival. It turns out that it departed 6min earlier, flight time is 12h15m, and 1h4m late arrival.
If you look at the flight history (link below), all previous flights took at most 11h15m, so this one is particularly long.
My questions are, what could be the delay factor (headwind?), and how come they can predict this before departure?
Thanks in advance.