I don't keep up with the good and bad of LEDs, so many being third world imports, etc. "The thing" about the dimmer is that it's not a voltage divider, it's a simple series rheostat. What this means is that the CURRENT LOAD on the rheostat establishes the output.
To put this another way, if you were to unhook all bulbs from the dimmer, and check it's output with a voltmeter, IT WOULD NEVER CHANGE. It would always remain "same as battery," because there is no load
My take, after reading some posts, is that "some" LED replacements don't put enough load on the dimmer to allow it to control.
And, as TB says, many LEDS have a sort of threshold
The "thing" about LEDs:
LEDs are, as the name implies, a DIODE. They are in operation when conducting, which in simple terms, turns them into a SHORT.
So why don't they destroy something? Because they require either a built in or separate CURRENT LIMITING resistor.
Once you increase the supply voltage so that the LED start -- and remains -- in conduction, and producing light, some of them don't change brilliance very much
