Int he Envelope the horizontal position is the input/phase, the vertical position is the output/param value.
The number you see there in the param thingy is the actual parameter value, but the slider is the phase percentage which controls the X position in the envelope. This can be confusing indeed.
In this shot, the param slider is at 50%, the small dot in the envelope is at 50% horizontal, the envelope says it needs to be at 90% vertical. This vertical position is your output value for the parameter.
There is narrow line for the parameter value on the slider at 90% of the param range:
Now, if you have a complex envelope like this one, there are multiple positions where you can get the 100% output value:
And this one you there is no position where you can get 100% scale as output:
In this case there is no way to reset the slider to a specific position which would represent 100% value position.
Does this explain it better?