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DGI assesses glare on the basis of contrast within the distribution of luminance within the field of view of the occupant, where high luminance parts of the view (sun or bright sky) contribute positively and background luminance (the general brightness of the overall image) contributed negatively to the DGI value. In other words: the higher the ambient light from artificial lighting/daylight the less bothered an occupant might be about a glare source such as the sun. I cannot follow the numbers you show but what I expect is that with more artificial lighting DGI values should be lower.

This presentation by Jan Wienold might help:

Note also the limitations of DGI as an indicator. Personally I would also be careful in using Energyplus for performance predictions of glare.