If i'm not mistaken, you can't override it except if you're willing to change the source code of jQm.
The line that are the source of that behavior are in the function refresh. More precisely the following block:
if ( typeof val === "object" ) {
data = val;
// a slight tolerance helped get to the ends of the slider
tol = 8;
left = this.slider.offset().left;
width = this.slider.width();
pxStep = width/((max-min)/step);
if ( !this.dragging ||
data.pageX < left - tol ||
data.pageX > left + width + tol ) {
return;
}
if ( pxStep > 1 ) {
percent = ( ( data.pageX - left ) / width ) * 100;
} else {
percent = Math.round( ( ( data.pageX - left ) / width ) * 100 );
}
}
In the case of an interraction with the mouse/tap, val is an object and in your case pxStep is inferior to 1. So we have the round coming into action.
P.S: Not totally sure about what I wrote, it just had a quick glance at the code , but it seems to me that it behave this way.