Lux calculation differs from sensor documentation

[adolfintel]

This library calculates lux with just this formula:

Adafruit_TCS34725/Adafruit_TCS34725.cpp

Line 513 in 6dc4283

illuminance = (-0.32466F * r) + (1.57837F * g) + (-0.73191F * b);

The sensor documentation here, in section 3.2 shows that an additional step is required to calculate the lux value: the calculated value needs to be divided by INTEGRATION_TIME_MS*SENSOR_GAIN/DGF, where DGF for this sensor is 310 assuming there is no glass in front of if (see appendix 1).

Is there a reason why the lux calculation was implemented this way?

Thanks