It is correct that you have a small amount of Green shift when measuring Sunlight.
Basically, what Lumu Power measures is chromaticity coordinates in the CIE 1931 chromaticity space.
Planckian locus is a theoretical line which represents Color Temperature values with zero Green or Magenta offset. However, there are very few light sources which have chromaticity exactly on the Planckian locus.
The next image shows the zoomed-in section of the chromaticity diagram with D50, D55, D65 and A standard illuminants marking:
On the average, when measuring daylight, you would get roughly between 10/1000 and 150/1000 of Green cast. It heavily depends on the sun's angle, your geolocation, weather, quality of the air, etc.
If you are interested in further theory you can read the basic theory on these links: