Why movieclips are moving at different speeds?

1

Can anybody tell me why these two movieclips are moving at different speeds? I would like them to be moving at the same speed (while increasing xspeed every second).

actionscript 2, flash mx.


var xspeed = 1;

countDown = function () {
     xspeed += .05;
};
timer = setInterval(countDown, 1000);

onEnterFrame = function () {
        mountains1._x -= xspeed/8;
        mountains2._x -= xspeed/8;
};