I just took a reading, here in St. Louis, with the Spectra Professional three days ago. The sky was a clear, cloudless blue, and I took the reading near noon. With the ISO (really, ASA on this old meter) set to 125, I got exactly 1/125 at f/16. That surprised me, because here in St. Louis it is more usually a "Sunny 11.5 rule." It would not surprise me if the reading were to drop another 1/2 stop or so between here and Minnesota. And if the reading had been taken earlier in the year, when the sun was lower in the sky, it would not surprise me if it were a bit lower still. Atmospheric conditions have an effect as well. Of course, Sunny 16 applies to subjects that are fully frontally lit. Surfaces facing at a 45 degree angle to the sun will require a smidgin more exposure, etc.
Add all that up, and that's why we need light meters! I mostly se sunny 16 to check my meter, not to finalize the exposure.