Each time you're receving the AJAX data, you're always adding it to the next dropdown. However, you're not clearing the dropdown before you do so, and therefore you get the previous items as well.
To fix it for the Type->Model dropdowns for example (as shown in the video), you should start your populateModel() with an empty() call like this:
function populateModel(xmlindata) {
var mySelect = $('#printermodel');;
mySelect.empty();