In ZK 8.0.3, when dragging a slider button, the value of the slider will be calculated based on slider width / current position (in px)
This cause the slider value not to match with the declared step value if the width in px is smaller than the number of steps between min and max slider values.
test with:

<slider id="sld" width="190px" minpos="0" maxpos="1000" step="10" />

The following override shows the intended result:
http://zkfiddle.org/sample/pv4vlc/3-Slider-snap-to-step