why change the information displayed depending of content in document.write command

Question

PLEASE some one could explain to me, why changing a string text to the begining at output line changes o treatment of variables.

//
var myNum1 = 3
var myNum2 = 10;
if (myNum1 < myNum2) {
alert(myNum1+myNum2+"ok");
document.write(myNum1+myNum2+"TextAFTER");
document.write("TextBEFORE"+myNum1+myNum2);

}
OUTPUT
13TextAFTERTextBEFORE310

thanks in advanced