Why does this dimmer switch have reduced wattage capability for LED / CFL bulbs?
Since dimmable LEDs don’t use active PFC for cost reasons, the peak/average current is much higher, called the crest factor, so conduction losses demand this de-rating of power limits in a Triac dimmer.

The last page gives examples of multiple dimmers on the side and why that causes further derating by how far “removed” from the load to line a configuration is possible. “Removed” here means shifted away from power input by cascaded dimmers.