JAVASCRIPT NOT DISPLAYING THE RIGHT VALUE

Question

Dear Code Masters,

Many thanks for all your support so far, i asked some question regarding javascript some days ago, though it wasn't answered but it gave me qlues that finally solve the issue.
but now, am faced with another kind of javascript problem, i would have solve this using C# ASP.NET code behind but i want performance in the user interface that is why i am using javascript.

looking at the javascript below kindly advice me why TextBox87 is displaying the value 6.

document.getElementById('<%=TextBox87.ClientID%>').value = 4;
document.getElementById('<%=TextBox103.ClientID%>').value = 1;
document.getElementById('<%=TextBox104.ClientID%>').value = 1;

document.getElementById('<%=TextBox87.ClientID%>').value = parseInt(document.getElementById('<%=TextBox87.ClientID%>').value) + parseInt(document.getElementById('<%=TextBox103.ClientID%>').value) + parseInt(document.getElementById('<%=TextBox104.ClientID%>').value);

many thanks