>>4253371It's the filter strength, just like with filters you put in front of the lens. Strong filters let less light through but are better at filtering. There's a mosaic of red, green and blue filters on top of the sensor. When designing it you can either make the filters strong but rob you of more stops of light, or weak but let more light through (at the expense of being worse at filtering out the non-matching colors). See pic related as an example.
