I think the "Sunny 16" rule is a bit misnamed.
The Sun shines with the same intensity over any place of the planet given a certain inclination over the horizon.
The rule is supposed to apply two hours after sunrise, two hours before sunset, (other say: with the sun more than 20° above the horizon) with clear sky, and - importantly - with the sun on your back, i.e. in front of the subject (with side sun you will want to open exposure a bit).
The rule is based on the fact that once the sun is high enough, the exposure will be basically the same throughout the day.
- Pollution can probably alter sunlight, but really not much unless you live in XIX century London, or XXI century Peking (Beijing);
- What complicates things is the reflectivity of the environment surrounding your subject.
I find that for a normal urban environment, with cobblestones and asphalt, EV14.5 ("sunny 13") is the right rule.
The sunny 16 rule was enunciated by somebody working in the US Navy, for "field" condition, with the sea reflecting sunlight on the boat, and light grey painted boats reflecting sunlight on the boat itself (paint makes a smooth surface which behaves a bit like a mirror).
I suppose the half a stop excess closing is due to these particular conditions.
In a "concrete jungle" environment the rule should be enunciated as sunny 13 IMO. But everybody goes on saying sunny 16 (possibly because there normally is no "13" on the aperture ring).