To benjiboy's point - I think the sunny 16's biggest variation is in our own judgement of the conditions - the only sure condition is "bright sun", as soon as there are clouds, the conditions get gray (pun intended).
As an inappropriate aside, the other day (I was using a d*****l camera) I was shooting in an empty church, lights low to balance intensity with late afternoon window lighting. ASA was 200, I was exposing manually with a 28PC Nikkor. Took a reading with my trusty old Sekonic L-398 Studio. The exposure was f/16 at 5 seconds, and although I had expected to make an adjustment, the image was perfect.
As a variation on Deming's well known saying, "In incident we trust, all others be careful."
I've always thought George that the human eye is a very poor instrument for judging changes in light intensity because it reacts to changes so imperceptibly without the person noticing them.