Ikea Tradfri bulbs brightness issue

The public code for that DTH is on github:

Near the bottom, it looks like some code to fiquire out the safe color range. Considering there is some rounding involved it is very possible the bulb is dimmer either by design or maybe a rounding error or firmware bug when attached to Smartthings.

You can try this DTH to see if the bulb performs as expected

This should get you rocking