Update: in the late afternoon (5-7pm) my Nest gets direct sunlight through the window. The black plastic of the Nest case seems excellent at absorbing all this light as heat, raising the thermostat some 8 degrees above the ambient air temperature.

After going back and forth with Nest tech support, it turns out that the Nest does include two ambient light sensors (one for visible light and one for IR) but they're apparently not attempting to do anything useful with those sensors, like try to model the degree to which the Nest might be heated by the ambient light. Instead, my Nest was predictably over-cooling the house and its reported ambient temperature matched precisely what my temperature gun measured on the surface of the Nest.

The workaround is that I've programmed in less cooling for that two hour range. It deeply disappoints me that my $40 Honeywell never had this problem but my $250 Nest does have the problem. Presumably, the cheap white plastic of the Honeywell reflects more light.