The definitive research was done by Loyd Jones and his team at Kodak in the 1930s and 1940s. the seminal paper on the subject of illuminance of daylight is Sunlight and Skylight as Determinants of Photographic Exposure. I. Luminous Density as Determined by Solar Altitude and Atmospheric Conditions. and II. Scene Structure, Directional Index, Photographic efficiency of Daylight, Safety Factors, and Evaluation of Camera Exposure, JOSA, vol 38 and 39, Feb 1948 and Feb 1948.
The mind numbing seventy or so page paper was so thorough that it was made into the ANSI exposure guide, ANSI PH2.7 - 1986. It is said that if a meter disagrees with the guide, the meter is most likely wrong. It may be a coincidence, but the first ANSI light meter standard and transparency speed standard came out at the same time as Jones paper, and Jones was on the ANSI committee on exposure meters.
The light meter has to determine three different film types (transparency, color neg, and b&w neg) using the same meter reading. All three are determined differently. The meter can only be precise with one film type and it has to assume the other two. Since exposure is most important with transparency, the exposure meter is geared specifically for the transparency. The meters indicated exposure was the same as the transparencies speed point. In recent years, there has been a small adjustment in transparency film speed.
A good question about Sunny 16 is what is defined as sunny - how many clouds, what solar altitude, what latitude, what altitude, etc?
Exposure is based on 10,200 footcandles with the meter pointed directly toward the sun or 7680 footcandles when the sun is at an approximate angle of between 40 to 45 degrees. The light reflecting off a middle gray card when the sun is at a solar angle of 40 to 45 degrees is 933 footcandles. At f/16 the light striking the film is ~ 2.4 footcandles at one second. With an ISO 125 film, the shutter is set to 1/125 and the amount of exposure is 0.0189 footcandles or (~ 0.064 meter candle seconds).