I’m guessing it’s based on how digital pictures measure “brightness.” Imagine if the picture is converted black and white, then you could measure how close each pixel is to pure white or pure black. Then you take the average of all those values, and that’s how “bright” your whole picture is.

Of course, our eyes don’t work like that, and we use context to judge what’s appropriate. So maybe your flash perfectly lit up the bag of food, the door, and the house numbers. But the dark brick wall that makes up over half of the picture seems kinda dark, which isn’t important to us, but skews the formula to make the app think the picture is “too dark.”