If I had to guess, it looks like google and yahoo track by time. Each nth second it plots a point, so it's likely to miss graphing the actual high or low of the day.
I thought this too, but looked at the daily high and daily low at Fidelity, and it matched the actual low on both the graphs (which were the same). I figure it's either some data blip, or maybe some kind of dark pool trading that was logged after hours.