Pricing seems to really depend on time-of-day, i.e. demand. I did the same thing on my LAX-AKL 2100-0725 and AKL-SFO 1550-0700–buying 2 hours prior to landing to clear out emails before short I-to-D connections. They probably figured out people who buy 1 or 2 hours tend to do that before landing, or that their American majority customers tend to buy 1-2 h before landing in a foreign country rather than returning home. I paid $8.49/2h before NZ but $3.99/2h before SFO. SFO-LAX <2 h was much more expensive!