Why does arduino use 20% frequency scailing for PWM

Indeed. Furthermore:

Product Status: Discontinued

Try something like:
https://ams.com/tcs3410

Or if you need more resolution and sensitivity:

Adafruit probably still sells modules based on TCS34725, but this sensor has also been discontinued by AMS. I bet Adafruit c.s. will migrate to TCS3410 or a similar product as well.