为什么我的scrollTop获得的值无法满足条件？

window.onload=function(){
        waterfall();

        window.onscroll=function(){
            
    var parent=document.getElementById("main");
    var boxs=getByClass(parent,"box");
    var scrollH=document.documentElement.scrollTop||document.body.scrollTop;
       var documentH=scrollH+document.documentElement.clientHeight;
    var    lastH=boxs[boxs.length-1].offsetTop+Math.floor(boxs[boxs.length-1].offsetHeight/2);
       /*763*/
     
        if(lastH-350<documentH){      //这里的lastH不减掉350，就无法进入if语句？？？
                                /*465*/

dataInt={"data":[{"src":"1.jpg"},{"src":"2.jpg"},{"src":"3.jpg"},{"src":"7.jpg"},{"src":"4.jpg"},{"src":"5.jpg"},{"src":"6.jpg"}]};

for(var i=0;i<dataInt.data.length;i++){
        var divB=document.createElement("div");
        divB.className="box";
        parent.appendChild(divB);
        var divP=document.createElement("div");
        divP.className="pic";

        divB.appendChild(divP);

        myimg=document.createElement("img");
        myimg.src="./images/"+dataInt.data[i].src;
        
        divP.appendChild(myimg);
}
waterfall();
}
}


}

function waterfall(){
    
var oParent=document.getElementById("main");
var boxs=getByClass(oParent,"box");
var boxW=boxs[0].offsetWidth;
var num=Math.floor(document.documentElement.clientWidth/boxW);
oParent.style.cssText="width:"+boxW*num+"px;margin:0 auto;";
  
    boxHarr=new Array();
    for(var i=0;i<boxs.length;i++){
        if(i<num){
            boxHarr.push(boxs[i].offsetHeight);
        }else{
           
            var minH=Math.min.apply(null,boxHarr);
            var minHindex=getminHindex(boxHarr,minH);
            boxs[i].style.position="absolute";
            boxs[i].style.top=minH+"px";
            boxs[i].style.left=minHindex*boxW+"px";
            //boxs[i].style.left=boxs[minHindex].offsetLeft+"px";
            boxHarr[minHindex]=boxHarr[minHindex]+boxs[i].offsetHeight;
        }
    }

}

function getByClass(parent,clsName){

    a=parent.getElementsByTagName('*');
    var arr=[];
    for(var i=0;i<a.length;i++){
        if(a[i].className==clsName){
            arr.push(a[i]);
        }
    }
    return arr;
}


function getminHindex(arr,minH){
    
    for(var i in arr){
        if(arr[i]==minH)
            return i;    
    }

}

减掉350或者其它数值以后，可以完全正常运行，但是用原值就无法进入IF语句？