Help with MouseWheel

Hi.
The code snippet below is for running thru an index of pics.
The problem I have is that the following code works perfectly locally but when I run it in a browser the mousewheel advances or declines the picture by 2 instead of 1?!?!
I sense that it probably has something to do with a browser setting but not sure.
Any help would be greatly appreciated.

//start code

var indexNumber:int = 0;
var currentNumPics:int = 10;

stage.addEventListener(MouseEvent.MOUSE_WHEEL, onMouseWheelEvent);

function onMouseWheelEvent(mw:MouseEvent):void {
if (mw.delta<0) {
if (indexNumber<=currentNumPics) {
indexNumber++;
trace(indexNumber);
}
}
if (mw.delta>=0) {
if (indexNumber!=0) {
indexNumber–;
trace(indexNumber);
}
}
}
//end code