you have not provided enough information how you use the send function, but I assume that you do not set mime type to specify you are sending form data
xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
the data sent are in this case encoded as you encode a query string
xhr.send("name=foo&value=bar");
otherwise it will not be interpreted as form data by Developer Tools.
jquery does majority of work for you in this regard.
Update: To answer explicitly what is the difference...
if a request (typically POST) has Content-type header set to application/x-www-form-urlencoded the body is expected to be in the form of a standard querystring with url-encoded key=value pairs joined by &. Form data section then shows the key-value parameters (when viewed parsed). This way was much more common in past because it is a default for HTML forms.
other cases are shown in Request payload section (and nowadays parsed for readability as well for common formats like JSON).
