vCam unequal scrolling

Hi there everyone, this is my first post on the forum and I hope I am able to learn a lot while I'm here. Starting by fixing this bug I have been having.

Actionscript Code:

_root.currentMouseX = 275;
_root.currentMouseY = 200;
_root.lastMouseX = 275;
_root.lastMouseY = 200;

onEnterFrame = function(){
    _root.currentMouseX = _root._xmouse;
    _root.currentMouseY = _root._ymouse;
        
    xMouseDiffPos = (_root.currentMouseX - _root.lastMouseX)*0.4;
    xMouseDiffNeg = (_root.lastMouseX - _root.currentMouseX)*0.4;
    
    yMouseDiffPos = (_root.currentMouseY - _root.lastMouseY)*0.4;
    yMouseDiffNeg = (_root.lastMouseY - _root.currentMouseY)*0.4;
    
    if(_root.currentMouseX < _root.lastMouseX){
        _root.viewBox._x -= xMouseDiffNeg;
    }
    
    if(_root.currentMouseX > _root.lastMouseX){
        _root.viewBox._x += xMouseDiffPos;
    }
    
    if(_root.currentMouseY < _root.lastMouseY){
        _root.viewBox._y -= yMouseDiffNeg;
    }
    
    if(_root.currentMouseY > _root.lastMouseY){
        _root.viewBox._y += yMouseDiffPos;
    }
    
    
    _root.lastMouseX = _root._xmouse;
    _root.lastMouseY = _root._ymouse;
}

So in terms of mathematics, and the logic in my head, this code should work perfectly. The scene has a center point. When I place my mouse over the center point, I also want the vCam to be centered. However this is not the case. After moving the mouse around for 30 seconds or so, there is a noticable shift in placements.

Unless I am missing something, each time I move the mouse away from the center, the amount moved by the vCam should be equaled when I move the mouse back.

My explanations might not be perfectly clear so I will attach an example .fla.

I appreciate all that can help with this problem, it has been bugging me for a while now.