Javascript getelementby tagname problem!

Question

im doing a clientside gridview calculation

 

in javascripti have used 

 

var gv = document.getElementById("<%= GridView1.ClientID %>");
           var tb = gv.getElementsByTagName("input");
           var lb = gv.getElementsByTagName("span");

and in the gridview i have a textbox, the calculation goes perfect but when i add select all checkbox the calculation comes right but total gets populated in alternate column.

 

when i remove the select all checkboxs it words fine but if i add the checkbox the total  gets populated in alternate column.