Oh, that explains it 
Blue has only 2 bits so that colors fit into 8 bits -- the human eye is least responsive to blue, so that's why red and green have more bits.
btw. the values of resistors are critical to get linear distribution of color bits. Resistors values are 806, 1.58K and 3.16K. Each value is approximately x2 the previous one. You can use other values as long as they follow the same pattern: x1, x2, x4.