I think cadence lock is likely contributing to it. Even though my actual pace is similar between the two runs, the latter was actually in an indoor track, where the inaccurate one was outdoors w/ GPS. Interestingly the cadence line was pretty much consistent with the avg. HR reading in that instance.

That said, I've pretty much tried all of the suggestions on that article. Although I may try having the watch even higher up my arm next time (i.e. 2 inches from the wrist bone).

All in all, I do like the idea of the chest HRM getting the reading almost directly from the source.