There are a few differences I can think of Why Kirby Gets Gold and Royal gets silver. Some of these may not apply to overall CRI rating.
1) Kirby has a more efficient airpath than Royal. Also, Kirby has a 7 amp motor, Royal needs 9 amps to achieve the same airflow.
2) Royal has standard F&G paper bag, while Kirby has cloth HEPA bag = better filtration.
3) Royal commercial has 10 amp motor instead of 9 amp and may also have cloth HEPA bag as standard giving it a Gold rating.)
4) Kirby has easy to operate Toe Touch height control, Royal has Adjust O Rite height control knob which requires you to bend down to adjust height.
5) Kirby has easy to use Belt Lifter which aids in belt and brushroll maintenance. Royal requires you to use separate belt removal tool or use your finger (not easy to do) to remove belt.