"Brightness variations" refer to changes in the amount of light or brightness levels. It means that there are shifts or fluctuations in the intensity of light, either increasing or decreasing. Full definition
It was the very first display to ever to get all Green (Very Good to Excellent) Ratings in all categories (except for Brightness variation with Viewing Angle, which is the case for all LCDs) since we started the Display Technology Shoot - Out article Series in 2006. (displaymate.com)
The Galaxy Tab S establishes new records for best Tablet display performance in: Highest Color Accuracy, Infinite Contrast Ratio, Lowest Screen Reflectance, and smallest Brightness Variation with Viewing Angle. (displaymate.com)
The Surface Pro 3 delivers uniformly consistent all around Top Tier display performance: it is only the second display to ever to get all Green (Very Good to Excellent) Ratings in all test and measurement categories (except for Brightness variation with Viewing Angle, which is the case for all LCDs) since we started the Display Technology Shoot - Out article Series in 2006, an impressive achievement for a display. (displaymate.com)