If you read the tooltip of StepSize you’ll notice it only works with keyboard/controller - you’re probably using mouse for this atm. StepSize allows you to use it as a variable for calculating new slider position when keyboard has the focus.
You’d need to write a bit of logic to have the mouse skip to the next value.
Besides, seeing 0.000001… is a standard float rounding error. See here for the best explanation ever in case you’re unfamiliar with the concept.
Hope it helps.