Paul, I believe your issue arises because of the way you are setting the value of your spinners when sliding. If you move too fast when sliding the slide
event can't keep up with you and as a result your spinner
values get behind which causes your issues.
if (ui.value > last) {
$("#spinners").spinner("stepUp", 1);
}
if (ui.value < last) {
$("#spinners").spinner("stepDown", 1);
}
So update your spinners like so:
$("#spinners").spinner("value", ui.value);
Now your spinners will always keep up with the latest slider value.