I've got sliding dimmers and I've noticed the same problem. My fixtures have 2 to 6 LED or CFL bulbs. Some don't even go very dark at all. My sliders switch off at the bottom of the slide though (sliding motion ends with the off switch). It's just that the minimum on setting is already somewhat bright and it varies with the type/number of bulbs. All the bulbs that I'm using are rated dimmable.